#include<iostream>
using namespace std;
 
void main()
{
	int grade;

	cin >> grade;

	switch(grade/10)
	{
		case 10:
		case 9:
			cout << "A" << endl;
			break;
		case 8:
			cout << "B" << endl;
			break;
		case 7:
			cout << "C" << endl;
			break;
		case 6:
			cout << "D" << endl;
			break;
		default:
			cout << "F" << endl;
	}
	system("pause");
}