Value NaN of a variable

Processing Beginners
4

Hi, this is the code that produces NaN:

void setup()
{
size(1000,1000);
background(255);
}
void draw()
{
  float x,y,xm,xM,ym,yM,A,B,inc,Zx,Zy;
  xm = -10; xM = 10; ym = -10; yM = 10;
  inc = (xM-xm)/1000;
  x = xm; y = ym;
  A = (x - xm)/(xM-xm)*1000;
  B = (y-ym)/(yM-ym)*1000;
  println(x,y,inc);
       for (int i = 0; i < 1000; i=i+1)
         {
            for (int k = 0; k < 1000; k++)
              {                    
               for (int l = 0; l < 20 ; l++)
                { println(x,y);
                  Zx = x*x-y*y;
                 Zy = 2*x*y;
           
                 if( Zx*Zx + Zy*Zy < 10)
                  {A = (x - xm)/(xM-xm)*1000;
                   B = (y-ym)/(yM-ym)*1000;  
                   stroke(l);
                   point(A,B);
                  }
                 x = Zx;
                 y = Zy;

               }
            y = ym + k*inc;
           
          }
           x = xm + i*inc;
          }
          exit();
}