Some task

#include<stdio.h>

long long s,n,k;

int main() {

    scanf("%lli %lli",&n,&k);
    s = 1;

    while (n>0) {

    if (s > 1e18/n) {

    s = -1;
    break;

    }

    s = s*n;
    n = n - k;

    }

    if (s>0)
    printf("%lli\n",s);
    else
    printf("overflow");

    return 0;
}