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

int main() {
    int a, b, c, d;
    cin >> a >> b >> c >> d;
    int cnt = 0;
    for (int i = a; i <= b; ++i) {
        for (int j = c; j <= d; ++j) {
            if (i != j) cnt++;
        }
    }
    cout << cnt << endl;
    return 0;
}