import java.util.*;
import java.lang.*;
import java.io.*;

// The main method must be in a class named "Main".
class Main {
    public static int fact(int n){
        if(n==0 || n==1) return 1;
        else return n*fact(n-1);
    }
    public static void main(String[] args) {
        System.out.println("Hello world!");
        int n=5;
        int res=fact(n);
        System.out.println(res);
    }
}