package main

import "fmt"

func main(){
	var a,b,c,d int
	fmt.Scan(&a, &b, &c, &d)
	ans := 0
	for i := a; i <= b; i++ {
		if (c <= i) && (i <= d){
			ans += d-c
		}else{
			ans += d-c+1
		}
	}
	fmt.Println(ans)
}