input = require('fs').readFileSync('/dev/stdin', 'utf8')
lines = input.split('\n')
line = lines.shift().split(' ')
H = line[0] * 1
W = line[1] * 1 
line = lines.shift().split(' ')
h = line[0] * 1
w = line[1] * 1 
console.log(H * W - h*W - w*H + h * w)
