#include <iostream>
#include <algorithm>

using namespace std;

int main() {
    int A, B, C, D;
    cin >> A >> B >> C >> D;

    int ans = 0;
    for (int i = A; i <= B; i++) {
        for (int j = C; j <= D; j++) {
            if (i != j) ans++;
        }
    }

    cout << ans << endl;
}