package algo;

import java.util.Scanner;

public class sample7 {

	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 cnt = 0;
		
		for(int i = A;  i <= B; i++){
			for(int j = C; j <= D; j++) {
				if(i != j) {
					cnt = cnt + 1;
				}
				else {
				}
			}
		}
		System.out.println(cnt);
	}
}