#include <bits/stdc++.h>

using namespace std;

int main() {
  int a, b, c, d;
  cin >> a >> b >> c >> d;
  int ans = 0;
  for (int i = a; i <= b; ++i) {
    for (int j = c; j <= d; ++j) {
      if (i != j) ans++;
    }
  }
  cout << ans << '\n';
  return 0;
}