#include <iostream>

using namespace std;

int main() {
    int a, b, c, d;
    cin >> a >> b >> c >> d;

    int ret = 0;
    for (int i = a; i <= b; i++) {
        for (int j = c; j <= d; j++) {
            if (i != j)ret++;
        }
    }
    cout << ret << endl;
    return 0;
}