function that adds and subtracts values from vars setting some of them to NaN

Question

I apologize for the length, but all of it's necessary, and I removed unimportant parts. The function:

function fight()
{
  var cellsLost, mitoLost, chloroLost = 0;
  var cellsGained, mitoGained, chloroGained = 0;
  var w = cells;
  var x = chloros;
  var z = mitos;

    if((difficulty == "easy")&&(lysos<=10))
    {
      cellsLost = randomInt(1,10-lysos); //-9 to 9
      mitoLost = Math.round(randomInt(0,4-((1/5)*lysos))); //-2 to 4
      chloroLost = Math.round(randomInt(0,8-((1/3)*lysos))); // -3 to 8
    }
    else if((difficulty == "easy")&&(lysos>10))
    {
      cellsGained = Math.abs((randomInt(1,10-lysos))); //
      mitoGained = Math.round(((1/5)*lysos) - randomInt(0,2));
      chloroGained = Math.round(((1/3)*lysos) - randomInt(0,3));
    }
    else if((difficulty == "average")&&(lysos<=20))
    {
      cellsLost = randomInt(0,20-lysos); //0 to 19
      mitoLost = Math.round(randomInt(0,10-((1/2)*lysos))); // -10 to 10
      chloroLost = Math.round(randomInt(0,15-((1/3)*lysos))); //-7 to 15
    }
    else if((difficulty == "average")&&(lysos>20))
    {
      cellsGained = Math.abs((randomInt(1,20-lysos)));
      mitoGained = Math.round(((1/5)*lysos)-randomInt(0,4));
      chloroGained = Math.round(((1/3)*lysos)-randomInt(0,7));
    }
    else if((difficulty == "challenging")&&(lysos<=30))
    {
      cellsLost = randomInt(0,30-lysos); //0 to 29
      mitoLost = Math.abs(Math.round(randomInt(0,15-((1/2)*lysos)))); //-15 to 15 ie 0-15 with double chances
      chloroLost = Math.round(randomInt(0,25-((1/3)*lysos))); //-10 to 25 ie 0 to 25 with double chances upto 10
    }
    else if((difficulty == "challenging")&&(lysos>30))
    {
      cellsGained = Math.abs((randomInt(1,30-lysos)));
      mitoGained = Math.round(((1/5)*lysos)-randomInt(0,6));
      chloroGained = Math.round(((1/3)*lysos)-randomInt(0,10));
    }

    mitoLost = negtozero(mitoLost);
    chloroLost = negtozero(chloroLost);
    cellsLost = negtozero(cellsLost);

    mitoGained = negtozero(mitoGained);
    chloroGained = negtozero(chloroGained);
    cellsGained = negtozero(cellsGained);

    cells = cells - cellsLost;
    mitos = mitos - mitoLost;
    chloros = chloros - chloroLost;

    mitos += mitoGained;
    chloros += chloroGained;
    cells += cellsGained;

    mitos = negtozero(mitos);
    cells = negtozero(cells);
    chloros = negtozero(chloros);

    divideCost = Math.round((15*(Math.pow(1.15,cells))));
    chloroCost = Math.round((100*(Math.pow(1.15,chloros))));
    mitoCost = (Math.round(150*(Math.pow(1.15,mitos))));

    display('The battle was long and hard.');
    if((w-cells)>0)
    {
      display('You lost ' + (w-cells) + ' cells.');
    }
    if((cells-w)>0)
    {
      display('You gained ' + (cells-w) + ' cells.');
    }
    if((x-chloros)>0)
    {
      display('You lost ' + (x-chloros) + ' chloroplasts.');
    }
    if((chloros-x)>0)
    {
      display('You gained ' + (chloros-x) + ' cells.');
    }
    if((z-mitos)>0)
    {
      display('You lost ' + (z-mitos) + ' chloroplasts.');
    }
    if((mitos-z)>0)
    {
      display('You lost ' + (mitos-z) + ' chloroplasts.');
    }
    attackCheck = setInterval(function() { cellAttack(); },1000);

    document.getElementById('fightbtn').style.display = "none";
    document.getElementById('fleebtn').style.display = "none";
  }
}

mitos, chloros and cells are all global vars previously initialized with values >0 or =0. Function negtozero() accepts an int argument and either returns the number if positive or 0 if it's negative. All the function does is provide a value to cellslost, chlorolost, mitolost and to cellsgained, mitogained, chlorogained based on certain conditions, and adds/subtracts themselves to cells, mitos and chloros respectively. I don't know why/how mitos and cells are being assigned NaN values, whereas chloros is being assigned the proper values.