C :

#include<stdio.h>
void main()
{
	int n;
	scanf("%d",&n);
	if(n>=20&&n<=30){
		printf("OK");
		
	}else{
		printf("NO");
		
	}
 } 

C++ :

#include<bits/stdc++.h>
using namespace std;
int main(){
   int t;
   cin>>t;
   if(t >= 20 && t <= 30){
   	  cout<<"OK"<<endl;
   }else{
   	  cout<<"NO"<<endl;
   }
   return 0;
}

Python :

t=int(input())
if t>=20 and t<=30:
    print('OK')
else:
    print('NO')