Problem:
Score : 100 points

Problem Statement

We have N+M balls, each of which has an integer written on it.

It is known that:

The numbers written on N of the balls are even.

The numbers written on M of the balls are odd.

Find the number of ways to choose two of the N+M balls (disregarding order) so that the sum of the numbers written on them is even.

It can be shown that this count does not depend on the actual values written on the balls.

Constraints

0 \leq N,M \leq 100

2 \leq N+M

All values in input are integers.

Input

Input is given from Standard Input in the following format:

N M

Output

Print the answer.

Sample Input 1

2 1

Sample Output 1

1

For example, let us assume that the numbers written on the three balls are 1,2,4.

If we choose the two balls with 1 and 2, the sum is odd;

If we choose the two balls with 1 and 4, the sum is odd;

If we choose the two balls with 2 and 4, the sum is even.

Thus, the answer is 1.

Sample Input 2

4 3

Sample Output 2

9

Sample Input 3

1 1

Sample Output 3

0

Sample Input 4

13 3

Sample Output 4

81

Sample Input 5

0 3

Sample Output 5

3

Solution in Rust (1.15.1) (judge verdict: Accepted)

#[allow(dead_code)]
mod utils {
    use std::io::{self, BufRead, BufReader, Stdin, StdinLock};
    use std::mem;
    use std::ops::{Deref, DerefMut};
    use std::str::{self, FromStr};

    pub struct Scanner<R: BufRead> {
        reader: R,
        buf: Vec<u8>,
    }

    impl<R: BufRead> Scanner<R> {
        pub fn new(reader: R) -> Scanner<R> {
            Scanner {
                reader: reader,
                buf: Vec::new(),
            }
        }

        pub fn read<T: FromStr>(&mut self) -> T {
            self.read_until(|&b| b == b' ' || b == b'\n');
            self.parse()
        }

        pub fn reads(&mut self) -> &str {
            self.read_until(|&b| b == b' ' || b == b'\n');
            unsafe { str::from_utf8_unchecked(&self.buf) }
        }

        pub fn readl(&mut self) -> &str {
            self.read_until(|&b| b == b'\n');
            unsafe { str::from_utf8_unchecked(&self.buf) }
        }

        pub fn read_vec<T: FromStr>(&mut self, count: usize) -> Vec<T> {
            let mut vec = Vec::with_capacity(count);

            for _ in 0..count {
                vec.push(self.read());
            }

            return vec;
        }

        fn read_until<P: FnMut(&u8) -> bool>(&mut self, mut predicate: P) {
            self.buf.clear();
            let mut first = true;

            // https://doc.rust-lang.org/src/std/io/mod.rs.html#1623-1649
            loop {
                let (done, used) = {
                    let available = self.reader.fill_buf().unwrap();
                    if first && available.len() == 0 {
                        panic!("EOF reached.");
                    }

                    match available.iter().position(&mut predicate) {
                        Some(0) if first => {
                            (false, 1)
                        }
                        Some(i) => {
                            self.buf.extend_from_slice(&available[..i]);
                            (true, i + 1)
                        }
                        None => {
                            self.buf.extend_from_slice(available);
                            (false, available.len())
                        }
                    }
                };

                self.reader.consume(used);
                if done || used == 0 {
                    return;
                }

                first = false;
            }
        }

        #[inline]
        fn parse<T: FromStr>(&mut self) -> T {
            // 文字列をT型へパースする
            let elem = unsafe { str::from_utf8_unchecked(&self.buf) };
            elem.parse()
                .unwrap_or_else(|_| panic!(format!("failed parsing: {}", elem)))
        }
    }

    pub struct StdinScanner {
        stdin: Stdin,
        scanner: Scanner<BufReader<StdinLock<'static>>>,
    }

    impl StdinScanner {
        pub fn new() -> StdinScanner {
            let stdin = io::stdin();
            let stdinlock = unsafe { mem::transmute(stdin.lock()) };
            let reader = BufReader::new(stdinlock);
            let scanner = Scanner::new(reader);
            StdinScanner {
                stdin: stdin,
                scanner: scanner,
            }
        }
    }

    impl Deref for StdinScanner {
        type Target = Scanner<BufReader<StdinLock<'static>>>;

        #[inline]
        fn deref(&self) -> &Self::Target {
            &self.scanner
        }
    }

    impl DerefMut for StdinScanner {
        #[inline]
        fn deref_mut(&mut self) -> &mut Self::Target {
            &mut self.scanner
        }
    }
}

fn main() {
    let mut scanner = utils::StdinScanner::new();
    let n: usize = scanner.read();
    let m: usize = scanner.read();

    println!("{}", n * n.saturating_sub(1) / 2 + m * m.saturating_sub(1) / 2);
}