#include <iostream>
#include <algorithm>
using namespace std;
int main() {
    int a, b, c, d;
    cin >> a >> b >> c >> d;
    int s = b - a + 1;
    int t = d - c + 1;
    int n1 = max(a, c);
    int n2 = min(b, d);
    int n = n1 - n2;
    if(n > 0) {
        cout << s * t << endl;
    } else {
        cout << (s * t + n - 1) << endl;
    }
}