#include <bits/stdc++.h>
using namespace std;

int main() {
    int a, b, c, d;
    cin >> a >> b >> c >> d;
    if (a > c) {
        swap(a, c);
        swap(b, d);
    }
    int ans = (b - a + 1) * (d - c + 1);
    if (b >= c) {
        ans -= min(b, d) - c + 1;
    }

    cout << ans << endl;
}