#include <bits/stdc++.h>
using namespace std;
using ll = long long;
using P = pair<int, int>;
using PL = pair<ll, ll>;
// Welcome to my source code!

int main() {
    int a, b, c, d;
    cin >> a >> b >> c >> d;
    ll ans = 0;
    for (int i = a; i <= b; i++) {
        for (int j = c; j <= d; j++) {
            if (i == j) continue;
            else ans++;
        }
    }
    cout << ans << endl;
}