Problem:
﻿Score: 100 points

Problem Statement

In 2020, AtCoder Inc. with an annual sales of more than one billion yen (the currency of Japan) has started a business in programming education.

One day, there was an exam where a one-year-old child must write a program that prints Hello World, and a two-year-old child must write a program that receives integers A, B and prints A+B.

Takahashi, who is taking this exam, suddenly forgets his age.

He decides to write a program that first receives his age N (1 or 2) as input, then prints Hello World if N=1, and additionally receives integers A, B and prints A+B if N=2.

Write this program for him.

Constraints

N is 1 or 2.

A is an integer between 1 and 9 (inclusive).

B is an integer between 1 and 9 (inclusive).

Input

Input is given from Standard Input in one of the following formats:

1

2
A
B

Output

If N=1, print Hello World; if N=2, print A+B.

Sample Input 1

1

Sample Output 1

Hello World

As N=1, Takahashi is one year old. Thus, we should print Hello World.

Sample Input 2

2
3
5

Sample Output 2

8

As N=2, Takahashi is two years old. Thus, we should print A+B, which is 8 since A=3 and B=5.

Solution in C# (Mono 4.6.2.0) (judge verdict: Accepted)

using System;
using System.Collections.Generic;
using System.Linq;
using System.IO;
using System.Text;
using System.Runtime.CompilerServices;
using static System.Console;
using static System.Math;
using static Ex;
using static Util;
using static IO_Z;

class AC
{

    /*------------------------------------------------------------------*/

    void Solve()
    {
        int n, a, b;
        I(out n);
        if (n == 1) { "Hello World".Prt(); }
        else
        {
            I(out a, out b);
            (a + b).Prt();
        }

    }

    /*------------------------------------------------------------------*/

    void Test()
    {
        //--- TEST ---//
    }

    public static void Main(string[] args) => new AC().Stream();

    private void Stream()
    {
        SetOut(IO_Z.StreamWriter);
        /*
        var exStdIn = new http://System.IO .StreamReader("stdin.txt");
        SetIn(exStdIn);
        Test();
        */
        Solve();
        Flush();
    }

}

static class IO_Z
{

    private static int Size = 1 << 12;
    private static readonly Stream Stream = OpenStandardInput();
    private static readonly byte[] Buffer = new byte[Size];
    private static int _ptr = 0, _buffLen = 0;

    [MethodImpl(MethodImplOptions.AggressiveInlining)]
    private static bool HasNextByte()
    {
        if (_ptr < _buffLen) return true;
        _ptr = 0;
        _buffLen = Stream.Read(Buffer, 0, Size);
        return _buffLen > 0;
    }

    [MethodImpl(MethodImplOptions.AggressiveInlining)]
    private static byte ReadByte()
    {
        if (HasNextByte()) return Buffer[_ptr++];
        else throw new EndOfStreamException("inputの末尾の改行がないとerror");
    }

    [MethodImpl(MethodImplOptions.AggressiveInlining)]
    private static bool IsAscii(int c) => '!' <= c && c <= '~';

    [MethodImpl(MethodImplOptions.AggressiveInlining)]
    private static char NextChar()
    {
        var b = ReadByte();
        while (!IsAscii(b)) b = ReadByte();
        return (char)b;
    }

    [MethodImpl(MethodImplOptions.AggressiveInlining)]
    private static string Next()
    {
        var sb = new StringBuilder();
        for (var b = NextChar(); IsAscii(b); b = (char)ReadByte()) sb.Append(b);
        return sb.ToString();
    }

    public static string Str => Next();
    public static char Cha => NextChar();
    public static int Int => int.Parse(Next());
    public static long Long => long.Parse(Next());
    public static double Double => double.Parse(Next());

    [MethodImpl(MethodImplOptions.AggressiveInlining)]
    private static void Scan <T>(ref T x)
    {
        if (x is int) __refvalue(__makeref(x), int) = int.Parse(Next());
        else if (x is long) __refvalue(__makeref(x), long) = long.Parse(Next());
        else if (x is double) __refvalue(__makeref(x), double) = double.Parse(Next());
        else if (x is char) __refvalue(__makeref(x), char) = NextChar();
        else __refvalue(__makeref(x), string) = Next();
    }

    [MethodImpl(MethodImplOptions.AggressiveInlining)]
    public static void I <T>(out T t)
    {
        t = default (T);
        Scan(ref t);
    }

    [MethodImpl(MethodImplOptions.AggressiveInlining)]
    public static void I <T, TU>(out T t, out TU u)
    {
        I(out t);
        I(out u);
    }

    [MethodImpl(MethodImplOptions.AggressiveInlining)]
    public static void I <T, TU, TV>(out T t, out TU u, out TV v)
    {
        I(out t, out u);
        I(out v);
    }

    [MethodImpl(MethodImplOptions.AggressiveInlining)]
    public static void I <T, TU, TV, TW>(out T t, out TU u, out TV v, out TW w)
    {
        I(out t, out u);
        I(out v, out w);
    }

    [MethodImpl(MethodImplOptions.AggressiveInlining)]
    public static void I <T, TU, TV, TW, TX>(out T t, out TU u, out TV v, out TW w, out TX x)
    {
        I(out t, out u, out v);
        I(out w, out x);
    }

    [MethodImpl(MethodImplOptions.AggressiveInlining)]
    public static void I <T, TU, TV, TW, TX, TY>(out T t, out TU u, out TV v, out TW w, out TX x, out TY y)
    {
        I(out t, out u, out v);
        I(out w, out x, out y);
    }

    public static void I <T>(out T[] a, int n)
    {
        a = new T[n];
        for (int i = 0; i < n; ++i) I(out a[i]);
    }

    public static void I <T, TU>(out T[] a, out TU[] b, int n)
    {
        a = new T[n];
        b = new TU[n];
        for (int i = 0; i < n; ++i) I(out a[i], out b[i]);
    }

    public static void I <T, TU, TV>(out T[] a, out TU[] b, out TV[] c, int n)
    {
        a = new T[n];
        b = new TU[n];
        c = new TV[n];
        for (var i = 0; i < n; ++i) I(out a[i], out b[i], out c[i]);
    }

    public static void I <T>(out T[,] a, int h, int w)
    {
        a = new T[h, w];
        for (int i = 0; i < h; ++i) for (int j = 0; j < w; ++j) I(out a[i, j]);
    }

    public static readonly StreamWriter StreamWriter = new StreamWriter(OpenStandardOutput())
    {
#if DEBUG
        AutoFlush = true
#else
        AutoFlush = false
#endif
    };

    public static void Flush() => Out.Flush();

    public static void Prt <T>(this T x) => WriteLine(x);

    public static void Prt <T>(this T[] xs) => xs.Iter(x => WriteLine(x));

}

static class Ex
{

    public static T[] Arr <T>(params T[] xs) => xs;

    public static void Iter <T>(this IEnumerable <T> xs, Action <T> f)
    {
        foreach (var x in xs) f(x);
    }

}

static class Util
{

    public static T IF <T>(bool b, Func <T> fTrue, Func <T> fFalse)
    {
        if (b) return fTrue();
        else return fFalse();
    }

    public static T LOCAL <T>(Func <T> f) => f();

    public static void FOR(int a, int b, int n, Action <int> f)
    {
        for (int i = a; i < b; i += n) f(i);
    }

    public static void FOR(int a, int b, Action <int> f) => FOR(a, b, 1, f);

    public static void REP(int a, Action <int> f) => FOR(0, a, f);

    public static void REP(int a, int b, Action <int, int> f) => REP(a, i => REP(b, j => f(i, j)));

    public static void REP(int a, int b, int c, Action <int, int, int> f) => REP(a, i => REP(b, j => REP(c, k => f(i, j, k))));

    public static T MIN <T>(params T[] xs) => xs.Min();

    public static T MAX <T>(params T[] xs) => xs.Max();

    public static IEnumerable <int> RANGE(int s, int e) => Enumerable.Range(s, e - s + 1);

}