using System;


public class ÇÐÁ¡Ã³¸®_switch
{
	public static void Main()
	{
		string strInput = "";
		string strGrade = String.Empty;
		string [] strArray;
		int intKor, intEng, intMat, intTot;
		double dblAvg;

		Console.WriteLine(
			"±¹¾î ¿µ¾î ¼öÇÐ Á¡¼ö¸¦ ÀÔ·ÂÇϽÿÀ.");
		strInput = Console.ReadLine().Trim();

		strArray = strInput.Split(' ');
		intKor = Convert.ToInt32(strArray[0]);
		intEng = Convert.ToInt32(strArray[1]);
		intMat = Convert.ToInt32(strArray[2]);

		intTot = intKor + intEng + intMat;
		dblAvg = intTot / 3.0;

		switch((int)dblAvg / 10)
		{
			case 10:
			case 9:
				strGrade = "A"; break;
			case 8:
				strGrade = "B"; break;
			case 7:
				strGrade = "C"; break;
			case 6:
				strGrade = "D"; break;
			default:
				strGrade = "F"; break;
		}

		Console.WriteLine("ÇÐÁ¡ : {0}", strGrade);
	}
}