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