package main

import "fmt"

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