#include <bits/stdc++.h>
#define exrep(i, a, b) for(long long i = a; i <= b; i++)
#define rep(i,n)for(long long i=0;i<(long long)(n);i++)
#define all(a) a.begin(), a.end()
#define rall(a) a.rbegin(), a.rend()
using namespace std;
typedef unsigned long long ull;
typedef long long ll;
typedef long double ld;
typedef pair<ll, ll> P;
typedef vector<ll> vl;
typedef vector<string> vs;
typedef vector<P> vp;
typedef vector<vector<ll> > vvl;
typedef vector<vector<P> > vvp;
template<class T> inline bool chmax(T& a, T b) { if (a < b) { a = b; return true; } return false; }
template<class T> inline bool chmin(T& a, T b) { if (a > b) { a = b; return true; } return false; }
 
const ll MOD=1e9+7;
const ll INF=1e18;
const int MAX=510000;
const double pi=acos(-1);
int dx[4]={1,0,-1,0};
int dy[4]={0,1,0,-1};

int main(){
  cin.tie(0);
  ios::sync_with_stdio(false); 
  
  int a,b,c,d;
  cin >> a >> b >> c >> d;
  int ans=(b-a+1)*(d-c+1);
  if(d<=a || b<=c) cout << ans << endl;
  else if(a<=c && d<=b) cout << ans-(d-c+1) << endl;
  else if(c<=a && b<=d) cout << ans-(b-a+1) << endl;
  else if(c<=a && a<=d && d<=b) cout << ans-(d-a+1) << endl;
  else cout << ans-(b-c+1) << endl;
  return 0;     
}