#include <bits/stdc++.h>
#define rep(i, ss, ee) for (int i = ss; i < ee; ++i)
using namespace std;

void solve() {
  int a, b, c, d;
  cin >> a >> b >> c >> d;
  set<int> s;
  rep(i, a, b + 1) s.emplace(i);
  rep(i, c, d + 1) s.emplace(i);
  int x = b + 1 - a;
  int y = d + 1 - c;
  cout << (x * y - (x + y - s.size())) << endl;
}

int main() {
  cin.tie(0);
  ios::sync_with_stdio(false);
  solve();
  getchar();
}