#include <iostream>
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 n = c - b;
    if(n > 0)
    {
        cout << s * t << endl;
    }
    else
        {
        cout << (s * t + n - 1) << endl;
    }
}