import java.util.*;

public class Main {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        int a = sc.nextInt();
        int b = sc.nextInt();
        int c = sc.nextInt();
        int d = sc.nextInt();
        int ans = (b - a + 1) * (d - c + 1);
        ans -= Math.max(0, b - c + 1);
        ans -= Math.max(0, a - d + 1);
        System.out.println(ans);
    }
}