#include <iostream>
using namespace std;
int main() {
  int a, b, c, d, cnt;
  cin >> a >> b >> c >> d;
  cnt = 0;
  for (int i = 1; i <= 100; i++) {
    if (a <= i && i <= b || c <= i && i <= d) {
      cnt++;
    }
  }
  cout << (b - a + 1) * (d - c + 1) - cnt << endl;
  return 0;
}