#include <iostream>

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;
    return 0;
}