/*
    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;
    int s;
    cin >> a >> b >> c >> d;

    if (a <= b && b < c && c <= d) {
        s = (b - a + 1) * (d - c + 1);
    } else if (a <= c && c <= b && b <= d) {
        s = (b - a + 1) * (d - c + 1) - (b - c + 1);
    } else if (c <= a && a <= d && d <= b) {
        s = (b - a + 1) * (d - c + 1) - (d - a + 1);
    } else {
        s = (b - a + 1) * (d - c + 1);
    }
    cout << s << endl;
}