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 = 0;
        for(int i = a; i <= b; i++) {
            for(int j = c; j <= d; j++) {
                if(i != j) {
                    ans++;
                }
            }
        }
        System.out.println(ans);
    }
}