package main

import . "fmt"

func main() {
	var a,b,c,d int
	Scan(&a,&b,&c,&d)
	ans:=0
	for u:=a;u<=b;u++ {
		for v:=c;b<=d;v++ {
			if u!=v {
				ans++
			}
		}
	}
	Println(ans)
}