package main

import "fmt"

func main() {
	var a, b, c, d, cnt int
	fmt.Scan(&a, &b, &c, &d)
	for i := a; i <= b; i++ {
		for j := c; j <= d; j++ {
			if i != j {
				cnt++
			}
		}
	}
	fmt.Println(cnt)
}