#include <bits/stdc++.h>
using namespace std;
using namespace chrono;
#if __has_include(<atcoder/all>)
#include <atcoder/all>
using namespace atcoder;
#endif

int main() {
    int64_t a, b, c, d;
    cin >> a >> b >> c >> d;

    int64_t dup = max<int64_t>(0, min(b, d) - max(a, c) + 1);

    int64_t m = b - a + 1;
    int64_t n = d - c + 1;

    int64_t ans = (m - dup) * (n - dup) + (m - dup) * dup + dup * (n - dup) +
                  dup * (dup - 1);

    cout << ans << endl;

    return 0;
}