#include <bits/stdc++.h>

using namespace std;

typedef unsigned long long ull;
typedef long long ll;

int main(){
	int a, b, c, d;
	cin >> a >> b >> c >> d;
	cout << (b-a+1)*(d-c+1)-max(min(b, d)-max(a, c)+1, 0) << endl;
	return 0;
}