#include <bits/stdc++.h>
#include <atcoder/all>
using namespace atcoder;
using namespace std;
using ll=long long;
using ld=long double;
ld pie=3.141592653589793;
ll mod=998244353 ;
ll inf=10000999999999900;
int main(){
    ll a,b,c,d;
    cin >> a >> b >> c >> d;
    ll ans=0;
    for (ll i = a; i <=b; i++)
    {
        for (ll j = c; j <=d; j++)
        {
            if (i!=j)
            {
                ans++;
            }
        }
    }
    cout << ans << endl;
}