import java.util.*;

class Yuki799{
	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 aka = b-a+1;
		int kuro = d-c+1;
		int dab = a<c?Math.max(0,b-c+1):Math.max(0,d-a+1);
		System.out.println(aka*kuro-dab);
	}
}