#include <iostream>
using namespace std;
int main()
{
    int a, b, c, d, p = 0;
    cin >> a >> b >> c >> d;
    if(c <= b && a <= d) p = b - c + 1;
    if(d <= a && c <= b) p = a - d + 1;
    cout << (b - a + 1) * (d - c + 1) - p << endl;
    return 0;
}