Back button on the Fragment activity

Question

I have Overwritted my back button on the Fragment Activity (I use a navigation drawer), so when i'm clicking on a link in the menu it's opening a Fragment and when I click on the back button it goes back in the Fragment Activity.

What I did :

Replace Fragment with another on back button

Now in one Fragment(A) I have a Form, and when i validate the Form it replace to an another Fragment(B), and when I click back button on this last Fragment(B) it goes on the Fragment(A) but I prefer to go back on the Fragment Activity, how to do it ?

Fragment Activity:

public class MainActivity extends AppCompatActivity implements FragmentDrawer.FragmentDrawerListener, Accueil.OnFragmentInteractionListener, DemandeGratuite.OnFragmentInteractionListener, NosOffres.OnFragmentInteractionListener {

    private static String TAG = MainActivity.class.getSimpleName();
    private Toolbar mToolbar;
    private FragmentDrawer drawerFragment;

    //Initialisation de l activite avec les donnees necessaires
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        mToolbar = (Toolbar) findViewById(R.id.toolbar);
        setSupportActionBar(mToolbar);
        getSupportActionBar().setDisplayShowHomeEnabled(true);
        drawerFragment = (FragmentDrawer) getSupportFragmentManager().findFragmentById(R.id.fragment_navigation_drawer);
        drawerFragment.setUp(R.id.fragment_navigation_drawer, (DrawerLayout) findViewById(R.id.drawer_layout), mToolbar);
        drawerFragment.setDrawerListener(this);
        // Affichage de la navigation
        //displayView(0);
        Fragment fragment = new Accueil();
        FragmentManager fragmentManager = getSupportFragmentManager();
        FragmentTransaction fragmentTransaction = fragmentManager.beginTransaction();
        fragmentTransaction.add(R.id.container_body, fragment);
        fragmentTransaction.commit();
    }


    @Override
    public boolean onCreateOptionsMenu(Menu menu) {
        //Ajout des items
        getMenuInflater().inflate(R.menu.menu_main, menu);
        return true;
    }

    @Override
    public boolean onOptionsItemSelected(MenuItem item) {
        int id = item.getItemId();
        if (id == R.id.action_settings) {
            return true;
        }
        return super.onOptionsItemSelected(item);
    }

    @Override
    public void onDrawerItemSelected(View view, int position) {
        displayView(position);
    }

    private void displayView(int position) {
        Fragment fragment = null;
        String title = getString(R.string.app_name);
        switch (position) {
            case 0:
                fragment = new Accueil();
                title = getString(R.string.title_accueil);
                break;
            case 1:
                fragment = new NosOffres();
                title = getString(R.string.title_nosoffres);
                break;
            case 2:
                fragment = new DemandeGratuite();
                title = getString(R.string.title_demandegratuite);
                break;
            case 3:
                fragment = new ContactezNous();
                title = getString(R.string.title_contact);
                break;
            case 4:
                fragment = new Actualites();
                title = getString(R.string.title_actu);
                break;
            case 5:
                fragment = new MentionsLegales();
                title = getString(R.string.title_mentions);
                break;
            case 6:
                fragment = new ValidationDemande();
                title = getString(R.string.title_mentions);
                break;
            default:
                break;
        }
        if (fragment != null) {
            FragmentManager fragmentManager = getSupportFragmentManager();
            FragmentTransaction fragmentTransaction = fragmentManager.beginTransaction();
            fragmentTransaction.replace(R.id.container_body, fragment);
            fragmentTransaction.addToBackStack("name");
            fragmentTransaction.commit();
            // libelle du toolbar
            TextView titlet;
            titlet = (TextView) findViewById(R.id.main_toolbar_title);
            titlet.setText(title);
            titlet.setTypeface(Typeface.createFromAsset(getAssets(), "fonts/GothamBook.ttf"));
        }
    }

    @Override
    public void onInteraction(int id) {
        displayView(id);
    }

    @Override
    public void onBackPressed() {
        FragmentManager mgr = getSupportFragmentManager();
        if (mgr.getBackStackEntryCount() == 0) {
            super.onBackPressed();
        } else {
            mgr.popBackStack();
        }
    }
}

Fragment A:

    public interface OnFragmentInteractionListener {
        void onInteraction(int id);
    }

    private OnFragmentInteractionListener mListener;

    @Override
    public void onAttach(Activity activity) {
        super.onAttach(activity);
        try {
            mListener = (OnFragmentInteractionListener) activity;
        } catch (ClassCastException e) {
            throw new ClassCastException(activity.toString() + " must implement OnFragmentInteractionListener");
        }
    }

    @Override
    public void onDetach() {
        super.onDetach();
        mListener = null;
    }

(...Form...)

mListener.onInteraction(6); (To go on Fragment B)

Fragment B:

It's on this Fragment that I want to go back on Main Activity on Back Button click.