#include<bits/stdc++.h>
#define int long long
#define matsuri pair<int,int>
//const int iris = 1e9+7;
const int iris = 998244353;
using namespace std;

void solve()
{
	int a,b,c,d;
	cin>>a>>b>>c>>d;
	int l=max(a,c), r=min(b,d);
	cout<<(b-a+1)*(d-c+1)-max(0ll, r-l+1)<<'\n';
}

signed main()
{
	ios::sync_with_stdio(0);
	cin.tie(0);

	int T=1;
	//cin>>T;
	while(T--)
		solve();

	return 0;
}