C :

#include<stdio.h>
#include<math.h>
int main()
{

int n,i;
scanf("%d",&n);
for(i=2;i<=sqrt(n);i++)
{     
if(n%i==0) {printf("NO");break;}
}
if(i>sqrt(n))printf("YES");
return 0;

}

C++ :

#include<iostream>
#include<cmath>
using namespace std;
int main()
{
  int x,a=2;
  cin>>x;
  while (a<=floor(sqrt(x))&&(x%a!=0))
  ++a;
  if (a>floor(sqrt(x)))
   cout<<"YES"<<endl;
  else cout<<"NO"<<endl;
  return 0;
}

Python :

# coding=utf-8
import math
a=int(input())
i=2
root=math.sqrt(a)
while(i<=root):
    if(a%i==0):
        break
    i=i+1

if(i>root):
    print("YES")
else:
    print("NO")