#include <bits/stdc++.h>

using  namespace std;

int main() {
    int a, b, c, d;
    int ans;
    
    cin >> a >> b >> c >> d;
    
    if (b < c || d < a) {
        ans = (b - a + 1) * (d - c + 1);
    } else if (c <= b && b <= d) {
        ans = (c - a) * (d - c + 1) + (b - c + 1) * (d - c);
    } else {
        ans = (a - c) * (b - a + 1) + (d - a + 1) * (b - a);
    }
    
    cout << ans << endl;
    
    return 0;
}