#include <iostream>

using namespace std;

int main() {
    int A, B, C, D;
    cin >> A >> B >> C >> D;
    int ans = (B - A + 1) * (D - C + 1);
    for (int i = 1; i <= 100; ++i) {
        if (A <= i && i <= B && C <= i && i <= D) --ans;
    }
    cout << ans << endl;
    return 0;
}