#include <iostream>
//#define NDEBUG
#include <cassert>
using namespace std;

char numericToLetter(int grade) 
{
  char letter;
  assert((0 < grade) && (grade <= 100));

  if (grade >= 90) letter = 'A';
  else if (grade >= 80) letter = 'b';
  else letter = 'F';
   

   // postcond: 'A' <= letter <= 'F'
  assert(('A' <= letter) && ('F' >= letter));
  return letter;
}


int main()

{
	
	cout << numericToLetter(85) << endl;

	return 0;

}