#include <bits/stdc++.h>
#define rep(i, n) for (int i = 0; i < n; ++i)
using ll = long long;
using namespace std;
const int INF = 1e9;

int main() {
    int a, b, c, d;
    cin >> a >> b >> c >> d;

    ll ans = (b - a + 1) * (d - c + 1);

    ans -= max(0, min(b, d) - max(a, c) + 1);
    cout << ans << endl;

    return 0;
}