c++ How to detect an input character as ENTER

Question

I'm trying to do a simple terminal calculator, but i'm stuck in a problem.

    #include <iostream>
using namespace std;

int main(){
    float n1, n2 = 0;
    char op;
    cout << "Value:     ";
    cin >> n2;       //as you can see, i MUST click enter to the number to be registered.
    while (true){
        cin.ignore(1);
        cout << "Operator:  ";
        system("stty raw");        
        op = getchar();           
        system("stty cooked");    
        cout << "\nValue:     ";
        cin >> n1;
        if(n1)
        {
            if(op == '+')
            {
            n2 = n2 + n1;
            cout << "           =\n           " << n2 << "\n";
            }
            if(op == '-')
            {
            n2 = n2 - n1;
            cout << "           =\n           " << n2 << endl;
            }
        }
        else
        {
            cout << "wrong."; //not really using it yet.
        }
    }
}

I want to read every character of keyboard input (think it's stdin), and when it reads the "+" operator, automatically get a new line and stop the keyboard input.

Like: im typing 5... 3... 4... and to choose the operator just click "+", not ENTER to run the number, and then choose the operator.

Hope i could explain.

Thanks in advance, i'm an extreme begginer in c++, so be nice pls :)