#include<iostream>

using namespace std;

int main(void)
{
	int i;

	cin >> i;

	switch(i/10)
	{
		case 10: 
		case 9:	cout << "A\n"; break;
		case 8:	cout << "B\n"; break;
		case 7:	cout << "C\n"; break;
		case 6:	cout << "D\n"; break;
		default: cout << "F\n"; break;
	}

	return 0;
}