I have a problem with casting.

I have a class Complex, where a Complex object is a complex number. The real and complex parts are floats. I sum several complex numbers, and I know that this sum is always an integer. I want to convert it to a long. The problem is that even though the sum is 11 , when I cast it to a long the result it 10. Why?

Code:

class Complex{
float r, c; //r is the real part, c the complex; r + ci
public:
Complex(){
r = 0;
c = 0;
}
Complex(float real, float comp){
r = real;
c = comp;
}
float getReal(){
return r;
}
float getComp(){
return c;
}
//additional irrelevant functions
}
long findKm(){
Comlex sum = //sum of several complex numbers that sums to a real number, which is an integer
long Km = static_cast<long>(sum.getReal())
return Km
}