/*
    No.799 赤黒かーどげぇむ
    https://yukicoder.me/problems/no/799
*/
#include <bits/stdc++.h>
using namespace std;

int main() {
    ios::sync_with_stdio(0);
    cin.tie(0);

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