Problem:
Score : 100 points

Problem Statement

There are H rows and W columns of white square cells.

You will choose h of the rows and w of the columns, and paint all of the cells contained in those rows or columns.

How many white cells will remain?

It can be proved that this count does not depend on what rows and columns are chosen.

Constraints

All values in input are integers.

1 \leq H, W \leq 20

1 \leq h \leq H

1 \leq w \leq W

Input

Input is given from Standard Input in the following format:

H W
h w

Output

Print the number of white cells that will remain.

Sample Input 1

3 2
2 1

Sample Output 1

1

There are 3 rows and 2 columns of cells. When two rows and one column are chosen and painted in black, there is always one white cell that remains.

Sample Input 2

5 5
2 3

Sample Output 2

6

Sample Input 3

2 4
2 4

Sample Output 3

0

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

#![allow(unused)]

use std::{cell, cmp, collections, fmt, ops};

use cell::{Cell, RefCell};
use cmp::Ordering::{self, Equal, Greater, Less};
use collections::{BTreeMap, BTreeSet, BinaryHeap, HashMap, HashSet, LinkedList, VecDeque};
use fmt::{Debug, Display};
use ops::{
    Add, AddAssign, Deref, DerefMut, Div, DivAssign, Mul, MulAssign, Neg, Not, Sub, SubAssign,
};

// https://qiita.com/tanakh/items/0ba42c7ca36cd29d0ac8
macro_rules! input {
    (src = $s:expr, $($r:tt)*) => {
        let mut iter = $s.split_whitespace();
        _input_inner!(iter, $($r)*)
    };
    ($($r:tt)*) => {
        let mut s = {
            use std::io::Read;
            let mut s = String::new();
            std::io::stdin().read_to_string(&mut s).unwrap();
            s
        };
        input!(src = s, $($r)*)
    };
}

macro_rules! inputln {
    ($($r:tt)*) => {
        let mut s = {
            use std::io::Read;
            let mut s = String::new();
            std::io::stdin().read_line(&mut s).unwrap();
            s
        };
        let mut iter = s.split_whitespace();
        _input_inner!(iter, $($r)*)
    };
}

macro_rules! _input_inner {
    ($iter:expr) => {};
    ($iter:expr, ) => {};

    ($iter:expr, $var:ident : $t:tt $($r:tt)*) => {
        let $var = _parse_value!($iter, $t);
        _input_inner!($iter $($r)*)
    };

    ($iter:expr, mut $var:ident : $t:tt $($r:tt)*) => {
        let mut $var = _parse_value!($iter, $t);
        _input_inner!($iter $($r)*)
    };
}

macro_rules! _parse_value {
    ($iter:expr, ( $($t:tt),* )) => {
        ( $(_parse_value!($iter, $t)),* )
    };

    ($iter:expr, [ $t:tt ; $len:expr ]) => {
        (0..$len).map(|_| _parse_value!($iter, $t)).collect::<Vec<_>>()
    };

    ($iter:expr, chars) => {
        _parse_value!($iter, String).chars().collect::<Vec<char>>()
    };

    ($iter:expr, usize1) => {
        _parse_value!($iter, usize) - 1
    };

    ($iter:expr, $t:ty) => {
        $iter.next().expect("No item left").parse::<$t>().expect("Parse error")
    };
}

#[cfg(feature = "debug")]
macro_rules! d {
    ($val:expr) => {
        match $val {
            tmp => {
                eprintln!("[{}] {} = {:#?}", line!(), stringify!($val), &tmp);
                tmp
            }
        }
    };
}

#[cfg(not(feature = "debug"))]
macro_rules! d {
    ($val:expr) => {
        $val
    };
}

macro_rules! vmin {
    ($x:expr, $y:expr) => { cmp::min($x, $y) };
    ($x:expr, $($args:expr),*) => { cmp::min($x, vmin!($($args),*)) };
}

macro_rules! vmax {
    ($x:expr, $y:expr) => { cmp::max($x, $y) };
    ($x:expr, $($args:expr),*) => { cmp::max($x, vmax!($($args),*)) };
}

fn main() {
    input!(height: usize, width: usize, h: usize, w: usize);
    let ans = (height - h) * (width - w);
    println!("{}", ans);
}

mod num {
    pub fn gcd(a: u64, b: u64) -> u64 {
        if b == 0 {
            a
        } else {
            gcd(b, a % b)
        }
    }

    pub fn gcd_list(a: &[u64]) -> u64 {
        a.iter().fold(a[0], |a, b| gcd(a, *b))
    }

    pub fn lcm(a: u64, b: u64) -> u64 {
        a * b / gcd(a, b)
    }

    pub fn lcm_list(a: &[u64]) -> u64 {
        a.iter().fold(a[0], |a, b| lcm(a, *b))
    }
}

mod bits {
    pub fn is_high(bits: u64, pos: u64) -> bool {
        bits & (1 << pos) != 0
    }

    /// len(0b1010) = 4
    /// len(0b1) = 1
    /// len(0b0) = 0
    pub fn len(bits: u64) -> usize {
        for b in 0..64 {
            if is_high(bits, 63 - b) {
                return (64 - b) as usize;
            }
        }

        return 0;
    }
}

#[derive(Clone, Debug)]
struct UnionFind {
    parent: Vec<usize>,
    size: Vec<usize>,
    depth: Vec<usize>,
}

impl UnionFind {
    fn new(capacity: usize) -> Self {
        UnionFind {
            parent: (0..capacity).collect(),
            size: vec![1; capacity],
            depth: vec![0; capacity],
        }
    }

    fn size(&mut self, a: usize) -> usize {
        let p = self.find(a);
        self.size[p]
    }

    fn find(&mut self, a: usize) -> usize {
        if self.parent[a] == a {
            a
        } else {
            let p = self.parent[a];
            self.parent[a] = self.find(p);
            self.parent[a]
        }
    }

    fn same(&mut self, a: usize, b: usize) -> bool {
        self.find(a) == self.find(b)
    }

    fn union(&mut self, a: usize, b: usize) {
        let a = self.find(a);
        let b = self.find(b);
        if a == b {
            return;
        }

        if self.depth[a] < self.depth[b] {
            self.parent[a] = b;
            self.size[b] += self.size[a];
        } else {
            self.parent[b] = a;
            self.size[a] += self.size[b];

            if self.depth[a] == self.depth[b] {
                self.depth[a] += 1;
            }
        }
    }
}

#[cfg(test)]
mod tests {
    use super::*;

    #[test]
    fn test() {
        //
    }
}