#include <bits/stdc++.h>
using namespace std;
#define rep(i, n) for (int i = 0; i < (n); i++)
typedef long long ll;
typedef pair<int, int> pii;
typedef vector<ll> vll;
typedef vector<int> vi;
typedef vector<vector<int>> vvi;
typedef vector<vector<ll>> vvll;
const ll inf = 1e16;
const ll md = 1000000007;

int main() {
  int a,b,c,d;
  cin>>a>>b>>c>>d;
  int com=max(b-c+1,0);
  int ans=(b-a+1)*(d-c+1)-com;
  cout<<ans<<endl;
  return 0;
}