#include <bits/stdc++.h>
using namespace std;
using ll = long long;
const ll mod = 1e9 + 7;
const ll inf = (1 << 30) - 1;
const ll infll = (1LL << 61) - 1;

int main()
{
    int a, b, c, d;
    cin >> a >> b >> c >> d;
    int e = 0;
    for (int i = a; i <= b; i++)
    {
        for (int j = c; j <= d; j++)
        {
            if (i == j)
                e++;
        }
    }
    cout << (b - a + 1) * (d - c + 1) - e << endl;
}