'use strict'

function main(input) {
  let a = input.split(/\n|\s/).map(v => ~~v)
  a = a.slice(0, -1)

  const grid = a[0] * a[1]
  const black = a[2] * a[1] + a[3] * a[0] - a[2] * a[3]
  console.log(grid - black)
}
main(require('fs').readFileSync('/dev/stdin', 'utf8'))