//neko.cpp
#include<iostream>
#include<string>
#include<math.h>
#include<stdlib.h>
#include<string>
#include<cstdio>
#include<stack>
using namespace std;
int main() {
	int a, b, c, d;
	cin >> a >> b >> c >> d;
	int sum = 0;
	for (int i = a; i <= b; i++) {
		for (int j = c; j <= d; j++) {
			if (i != j) {
				sum = sum + 1;
			}
		}
	}
	cout << sum;
}