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

// The main method must be in a class named "Main".
interface Calculator{
    int add(int a,int b);
}
class Main {
    public static void main(String[] args) {
        System.out.println("Hello world!");
        Calculator c=(x,y)->x+y;
        int output=c.add(3,5);
        System.out.println(output);
        
    }
}