C++ :

#include<bits/stdc++.h>
using namespace std;
long long n,a[1000000];
int main()
{
	a[1]=1;
	cin>>n;
	for(int i=2;i*i<=n;i++)
	{
		if(a[i]==0)
		{
			for(int j=i+i;j<=n;j+=i)
			    a[j]=1;
		}
	}
	if(a[n])
	    cout<<"No";
	else
	    cout<<"Yes";
	
	return 0;
}

Pascal :

var
  a,i:longint;
  b:boolean;
begin
  b:=false;
  readln(a);
  for i:=2 to a-1 do
  if a mod i=0 then begin
  b:=true;
  break;
  end;
  if (b) or (a=1) then write('No')
  else write('Yes');
end.