#include <iostream>
using namespace std;

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