// No.799 赤黒かーどげぇむ
package main

import "fmt"

func main() {
	var a, b, c, d int
	fmt.Scan(&a, &b, &c, &d)

	red := (b - a) + 1
	black := (d - c) + 1
	fmt.Println((red * black) - max((min(b, d)-max(a, c))+1, 0))
}