#include <stdio.h>
#include <stdlib.h>
#include <math.h>

#define NUMBER 600851475143

int isitprime(unsigned long int);

int main()
{
    unsigned long int x = 1;
    unsigned int largestprime = 42;
    int y;

    for (x = 1; x <= sqrt(NUMBER); x++)
        {
            printf("Testing %li \n", x);

            if ((NUMBER % x) == 0)
                if ((y = isitprime (x)))
                    largestprime = x;
        }

    printf("Largest prime is: %i", largestprime);
    return 0;
}