package main

import "fmt"

func main() {
	var mnR, mxR, mnB, mxB int
	fmt.Scan(&mnR, &mxR, &mnB, &mxB)

	cnt := 0
	for r := mnR; r <= mxR; r++ {
		for b := mnB; b <= mxB; b++ {
			if r != b {
				cnt++
			}
		}
	}
	fmt.Println(cnt)
}