#include <bits/stdc++.h>

using namespace std;
using llong = long long;
using ldbl = long double;
using P = pair<llong, llong>;

#define BE(x) x.begin(), x.end()

const llong inf = llong(1e18)+7;
const llong mod = 1e9+7;

int main(){
	int a,b,c,d;
	cin >> a >> b >> c >> d;
	llong ans = 0;
	for(int i = a; i <= b; i++)
		for(int j = c; j <= d; j++)
			if(i != j)
				ans++;

	cout << ans << endl;

	return 0;
}