#include<bits/stdc++.h>
using namespace std;
typedef long long ll;
#define F first
#define S second
#define pii pair<int, int>
#define eb emplace_back
#define all(v) v.begin(), v.end()
#define rep(i, n) for (int i = 0; i < n; ++i)
#define rep3(i, l, n) for (int i = l; i < n; ++i)
#define chmax(a, b) a = max(a, b)
#define chmin(a, b) a = min(a, b)
#define out(a) cout << a << endl
#define SZ(v) (int)v.size()
#define inf (int)(1e9+7)
int main() {
    int a, b, c, d;
    cin >> a >> b >> c >> d;
    //選ぶ数がかぶるということは同じ数を選ぶ
    //その数の選択肢は範囲が被ってるとこ
    out((b - a + 1) * (d - c + 1) - max(0, min(b, d) - max(a, c) + 1));
}