import java.util.Scanner;
class Main
{
    public static void main(String args[])
    {
	int m,f,r;
	Scanner scan=new Scanner(System.in);
	while(true)
	    {
		m=scan.nextInt();
		f=scan.nextInt();
		r=scan.nextInt();
		if((m==-1) && (f==-1) && (r==-1))
		    
			
			break;
		    
		else if((m==-1) || (f==-1))System.out.println("F");
		else if(m+f>=80)System.out.println("A");
		else if((m+f>=65) && (m+f<80))System.out.println("B");
		else if(((m+f>=50) && (m+f<65)) || (r>=50))System.out.println("C");
		else if((m+f>=30) && (m+f<50))System.out.println("D");
		else if(m+f<30)System.out.println("F");
	    }
    }
}
/tmp/hf-datasets-cache/medium/datasets/42704881117311-config-parquet-and-info-qiankunmu-Project_CodeNet-896c9d36/hub/datasets--qiankunmu--Project_CodeNet_Python800_and_Java250/snapshots/c899fde44a77ba1cc14973ed3de064e4ff3aadc7/Project_CodeNet_Java250.tar.gz