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 Rust (1.15.1) (judge verdict: Accepted)

#[allow(unused_imports)]
use template::buf_scan::*;
#[allow(unused_imports)]
use template::*;

#[allow(dead_code)]
#[macro_use]
pub mod macros {
    #[macro_export]
    macro_rules! debug {
        ($($a:expr),*) => {{
            use ::std::io::Write;
            writeln!(&mut ::std::io::stderr(), concat!($(stringify!($a), " = {:?}, "),*), $($a),*).unwrap();
        }}
    }

    #[macro_export]
    macro_rules! input {
        (source = $s:expr, $($r:tt)*) => {
            let mut scanner = $crate::template::buf_scan::BufScanner::new($s.as_bytes());
            input_inner!{scanner, $($r)*}
        };
        (scanner = $s:ident, $($r:tt)*) => {
            input_inner!{$s, $($r)*}
        };
        ($($r:tt)*) => {
            let stdin = ::std::io::stdin();
            let mut scanner = $crate::template::buf_scan::BufScanner::new(stdin.lock());
            input_inner!{scanner, $($r)*}
        };
    }

    macro_rules! input_inner {
        ($scanner:expr) => {};
        ($scanner:expr, ) => {};
        ($scanner:ident, $var:ident : $t:tt $($r:tt)*) => {
            let $var = read_value!($scanner, $t);
            input_inner!{$scanner $($r)*}
        };
        ($scanner:ident, mut $var:ident : $t:tt $($r:tt)*) => {
            let mut $var = read_value!($scanner, $t);
            input_inner!{$scanner $($r)*}
        };
    }

    macro_rules! read_value {
        ($scanner:ident, ( $($t:tt),* )) => {
            ( $(read_value!($scanner, $t)),* )
        };
        ($scanner:ident, [ $t:tt ; $len:expr ]) => {
            (0..$len).map(|_| read_value!($scanner, $t)).collect::<Vec<_>>()
        };
        ($scanner:ident, chars) => {
            read_value!($scanner, String).chars().collect::<Vec<char>>()
        };
        ($scanner:ident, usize1) => {
            read_value!($scanner, usize) - 1usize
        };
        ($scanner:ident, $t:ty) => {
            ($scanner).next::<$t>()
        };
    }
}

fn main() {
    let stdin = std::io::stdin();
    let mut stdin = stdin.locked_scanner();
    input! {
        scanner = stdin,
        i: usize,
    }
    if i == 1 {
        println!("Hello World");
    } else {
        input! {
            scanner = stdin,
            a: i32,
            b: i32,
        }
        println!("{}", a + b);
    }
}

#[allow(dead_code)]
pub mod template {
    pub mod buf_scan {
        pub trait LockRead<R: ::std::io::BufRead> {
            fn locked_scanner(self: Self) -> BufScanner<R>;
        }
        impl <'a> LockRead<::std::io::StdinLock<'a>> for &'a ::std::io::Stdin {
            fn locked_scanner(self: Self) -> BufScanner<::std::io::StdinLock<'a>> {
                BufScanner::new(self.lock())
            }
        }
        pub struct BufScanner<R: ::std::io::BufRead> {
            br: R,
            buf: Vec<u8>,
            pos: usize,
        }

        impl<R: ::std::io::BufRead> BufScanner<R> {
            pub fn new(br: R) -> Self {
                BufScanner {
                    br: br,
                    buf: Vec::new(),
                    pos: 0usize,
                }
            }
            fn fill_line(&mut self) {
                self.buf.clear();
                self.pos = 0;
                self.br
                    .read_until(b'\n', &mut self.buf)
                    .expect("Read a line");
                if let Some(&b'\n') = self.buf.last() {
                    self.buf.pop();
                    if let Some(&b'\r') = self.buf.last() {
                        self.buf.pop();
                    }
                }
            }
            fn skip_space(&mut self) {
                loop {
                    for i in self.pos..self.buf.len() {
                        if self.buf[i] != b' ' {
                            self.pos = i;
                            return;
                        }
                    }
                    self.fill_line();
                }
            }
            pub fn read_token(&mut self) -> &str {
                self.skip_space();
                let start = self.pos;
                while self.pos < self.buf.len() && self.buf[self.pos] != b' ' {
                    self.pos += 1;
                }
                let ret = unsafe { ::std::str::from_utf8_unchecked(&self.buf[start..self.pos]) };
                self.pos += 1; // skip the last space (or len() + 1 which is fine)
                return ret;
            }
            pub fn next<T: ::std::str::FromStr>(&mut self) -> T {
                self.read_token().parse::<T>().ok().expect("Parse")
            }
        }
    }
}