kombinacne cisla bez pretekania

#include <stdio.h>

int main() {
    unsigned long long int res = 1;
    int n, k, n_iterator, pom=1, flag = 1;
    while(scanf("%d %d", &n, &k)>0){
        if(k>=0 && n>=k){
            n_iterator = n;
            while (n_iterator != 1 || flag){
                res = res*n_iterator/pom;
                if(n_iterator >1)
                    n_iterator--;
                if(pom < k && flag==1){
                    pom++;
                }
                else if (flag==1) {
                    pom = 1;
                    flag = 2;
                }
                else if (pom < n-k && flag==2){
                    pom++;
                }
                else {
                    pom = 1;
                    flag = 0;
                }
            }
            printf ("%lu\n", res);
        } else printf ("zly vstup\n");
    }
}