#include <stdio.h>
int isprime(int x)
{
    int n = sqrt(x);
    for(int i = 2; i <= n; i++){
        if(x % i == 0) return 0;
    }
    return 1;
}

int main()
{
    int n;
    while(~scanf("%d",&n))
    {
        if(n < 2)   printf("No\n");
        else{
            if(isprime(n))  printf("Yes\n");
            else printf("No\n");
        }
    }
    return 0;
}

埃式筛
#include<bits/stdc++.h>
using namespace std;
bool vis[10000005]={true,true};
int main(){
	unsigned long long n;
	for(int i=2;i*i<=10000005;i++){
		if(!vis[i]){
			for(unsigned long long j=i*2;j<=10000005;j+=i){
				vis[j]=true;
			}
		}
	}
	while(cin>>n){
		if(!vis[n]){
			cout<<"Yes"<<endl;
		}
		else{
			cout<<"No"<<endl;
		}
	}
	return 0;
}