#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
typedef pair<ll, ll> l_l;

#define EPS (1e-7)
#define INF (1e9)
#define PI (acos(-1))
//const ll mod = 1000000007;

int main() {
    //cout.precision(10);
    cin.tie(0);
    ios::sync_with_stdio(false);
    int a, b, c, d;
    cin >> a >> b >> c >> d;
    int same = min(b, d) - max(a, c) + 1;
    if(a > d || c > b) same = 0;
    cout << (b - a + 1) * (d - c + 1) - same << endl;
    return 0;
}