import java.io.*;
import java.util.Scanner;


public class Main_yukicoder799 {

	private static Scanner sc;
	private static Printer pr;

	private static void solve() {
		int a = sc.nextInt();
		int b = sc.nextInt();
		int c = sc.nextInt();
		int d = sc.nextInt();

		int ans = 0;
		for (int i = a; i <= b; i++) {
			if (i >= c && i <= d) {
				ans += d - c;
			} else {
				ans += d - c + 1;
			}
		}
		
		pr.println(ans);
	}

	// ---------------------------------------------------
	public static void main(String[] args) {
		sc = new Scanner(System.in);
		pr = new Printer(System.out);
			
		solve();
			
		pr.close();
		sc.close();
	}

	static class Printer extends PrintWriter {
		Printer(OutputStream out) {
			super(out);
		}
	}
}