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

long max(long a, long b) {
    return a > b ? a : b;
}

long largest_prime_factor(long n) {
    int p = 2;
    while(n > 1) {
        while(p < n && n%p!=0) p++;
        while(n%p==0) n/=p;
    }
    return p;
}

int main( int argc, char *argv[] ) {
    assert(largest_prime_factor(13195) == 29);

    printf("%d\n",largest_prime_factor(600851475143));

	return 0;
}