//
//#define _GLIBCXX_DEBUG
//#define NDEBUG
#include <bits/stdc++.h>

using namespace std;

typedef long long ll;
const ll linf = 1e18;
const int inf = 1e9;

#define vl vector<ll>
#define pg priority_queue<ll, vector<ll>, greater<ll>>
#define pq priority_queue<ll>
#define rp(i, a, n) for (int i = a; i < n; i++)
#define pr(i, a, n) for (int i = a; i >= n; i--)
#define fm(i, mp) for (auto i = (mp).begin(); i != (mp).end(); ++i)
#define al(v) (v).begin(), (v).end()
#define mz(x) memset(x, 0, sizeof(x))
#define mi(x) memset(x, 0x3F, sizeof(x))
#define ce(x) cout << (x) << endl
#define cs(x) cout << (x) << ' '
#define co(x) cout << (x)
#define cd(x, y) cout << setprecision(x) << (y) << endl
#define ma make_pair
#define pb push_back
#define fs first
#define sc second
#define ceo cout << endl
#define cso cout << ' '
clock_t startT, endT;
int time()
{
    endT = clock();
    cerr << (endT - startT) / 1000 << "ms" << endl;
    return (endT - startT) / 1000;
}

int main()
{
    startT = clock();
    cin.tie(0);
    ios::sync_with_stdio(0);
    //----------------------------
    ll a, b, c, d;
    cin >> a >> b >> c >> d;
    ce((b - a + 1) * (d - c + 1) - max(ll(0), b - c + 1) - max(ll(0), a - d + 1));
    //----------------------------

    time();
    return 0;
}
//