import java.util.Scanner;

public class CardGame {
	public static void main(String args[]) {
		Scanner scan = new Scanner(System.in);
		int a = scan.nextInt();
		int b = scan.nextInt();
		int c = scan.nextInt();
		int d = scan.nextInt();
		int cnt = 0;

		for(int i = a; i <= b; i++) {
			for(int j = c; j <= d; j++) {
				if(i != j) {
					cnt++;
				}
			}
		}

		System.out.println(cnt);
		return;
	}
}