#include <iostream.h>


void main (void) {

  int people, i;
  const int days_in_leap_year = 366;
  double probability_of_no_shared_birthdays, probability_of_shared_birthday;

  cout << "Enter number of people at party (zero or -ve to stop): ";
  cin >> people;

  while (people > 0) {

    if (people > days_in_leap_year) {

      cout << "It is certain that two people will share a birthday."
           << endl;

    } else {

      probability_of_no_shared_birthdays = 1.0;
      i = 1;
      while (i < people) {
        probability_of_no_shared_birthdays *=
            (double(days_in_leap_year - i) / days_in_leap_year);
        i++;
      }
      probability_of_shared_birthday = 1.0 - probability_of_no_shared_birthdays;

      if (probability_of_shared_birthday > 0.999999) {

        cout << "It is almost certain that two people will share a birthday."
             << endl;

      } else {

        cout << "There is a " << probability_of_shared_birthday * 100
             << "% chance that two people will share a birthday."
             << endl;

      }

    }

    cout << "Enter number of people at party (zero or -ve to stop): ";
    cin >> people;

  }

  // we don't need to pause here (even if using the system in the lab)
  // because there's no reason to keep the screen around.

}