Largest Prime Factor- C++

I'm trying to find the largest prime factor of the number 600851475143. My code works for smaller numbers that I test (below 100). However when confronted with 600851475143, it returns 4370432, definitely not prime. Any ideas what could be wrong with my code?

#include <iostream>
#include <time.h>
#include <math.h>

using namespace std;

int main()
{

int num;
int largest;
int count;

cout<<"Please enter a number to have its Largest Prime Factor found"<<endl;
cin>>num;
num = 600851475143;

for (int factor = 1; factor <= num; factor++)
{
    if (num % factor == 0)
    {
        count = 0;
        for (int primetest=2; count == 0 && factor > primetest ; primetest++) 
        {
            if (factor % primetest == 0)
            count ++;    
            //endif
        }
        if (count == 0)
        largest = factor;
        //endif
    }       

}//endif
cout<<largest<<endl;
system("PAUSE");
}

num = 600851475143;

Integer overflow occurs here. The size of num is not large enough to contain the value which you've provided.

Use uint64_t.

#include <cstdint>  //must include this!

uint64_t num = 600851475143;

Read this : cstdint