#include<iostream>
#include<iomanip>
using namespace std;

void main()
{
	int n;		//4 bytes, whole numbers
	float f;	//4 bytes, floating point numbers
	char c;		//1 byte, character
	char c2;

	c = 'a';
	c2 = c;

	//cout << "Is today Friday (y/n) ?  ";
	//cin >> c;

	//if(c == 'y' || c == 'Y')
	//{
	//	cout << "Great\n";
	//}
	//else if(c == 'n' || c == 'N')
	//{
	//	cout << "Too bad\n";
	//}
	//else
	//{
	//	cout << '"' << c << "\" is not a valid choice\n";
	//}

	//if(c >= 'a' && c <= 'k')

	//for(int i = 0; i<=255; i++)
	//{
	//	//type cast
	//	cout << i << "\t" << (char) i<< endl;
	//}

	int grade;
	cin >> grade;

	switch(grade/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;
	}


	//if(grade > 100)
	//{
	//	cout << "Over achiever\n";
	//}
	//else if(grade >= 90)
	//{
	//	cout << "A\n";
	//}
	//else if (grade >= 80)
	//{
	//	cout << "B\n";
	//}
	//else if (grade >= 70)
	//{
	//	cout << "C\n";
	//}
	//else if (grade >= 60)
	//{
	//	cout << "D\n";
	//}
	//else 
	//{
	//	cout << "F\n";
	//}
}