#include <iostream>

using namespace std;

int main() {
    int a, b, c, d;
    cin >> a >> b >> c >> d;
    int t = (b - a + 1) * (d - c + 1);
    if (c < b) {
        t -= b - c + 1;
    }
    cout << t << endl;

    return 0;
}