package atcoder;

import java.util.ArrayList;
import java.util.Scanner;

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();

		sc.close();

		int ans = 0;

		ArrayList<Integer> Red = new ArrayList<>();
		ArrayList<Integer> Black = new ArrayList<>();

		for(int i=a; i<=b;i++)Red.add(i);
		for(int i=c; i<=d;i++)Black.add(i);

		for (int i=0; i<Red.size(); i++){
			for(int j=0; j<Black.size(); j++){
				if (Red.get(i)!=Black.get(j)) ans++;
			}
		}

		System.out.println(ans);

	}

}