#include <bits/stdc++.h>
using namespace std;
int main()
{
    int a, b, c, d, ans = 0;
    cin >> a >> b >> c >> d;
    for (int i = a; i <= b; i++) for (int j = c; j <= d; j++) if (i != j) ans++;
    cout << ans << endl;
    return 0;
}