Annoying problem

[spikeyz]

I have this code:

int x,y;
long a=0,b=0,dx=0,dy=0;
dx=x1-x0;
dy=y1-y0;
a=dy/dx;
b=y0-a*x0;

When dx is larger than dy In the last line 'a' is always. In debug it acts like a is an Int. I don't get it.
Why?