#include <bits/stdc++.h>
using namespace std;
using ll = long long;

int a, b, c, d;

int main()
{
    cin >> a >> b >> c >> d;
    int ans = 0;
    for (int i = a; i <= b; i++)
    {
        for (int j = c; j <= d; j++)
        {
            if (i != j)
                ans++;
        }
    }
    cout << ans << endl;
}