#include <bits/stdc++.h>
using namespace std;

int main() {
  int a, b, c, d;
  cin >> a >> b >> c >> d;
  if (b < c || d < a) {
    cout << (b - a + 1) * (d - c + 1) << endl;
  }
  else {
    cout << ((b - a + 1) * (d - c + 1) - (min(b, d) - max(a, c) + 1)) << endl;
  }
  return 0;
}