http://e-tutor.itsa.org.tw/e-Tutor/mod/programming/view.php?id=30752
#include<stdio.h> int main(){ int n,i; scanf("%d",&n); for(i=2;i<n;i++) if(n%i==0)break; if(i==n||i==0)printf("YES\n"); else printf("NO\n"); }