

import java.util.*;

public class Main {
	public static void main(String args[]){
		Scanner scan = new Scanner(System.in);
		StringBuilder sb = new StringBuilder();
		int m,f,r;
		//?????????????¨????
		m = 0;
		f = 0;
		r = 0;
		
		while(true){//??\???????????¨???-1????????£??????????????????
			
			m = scan.nextInt();
			f = scan.nextInt();
			r = scan.nextInt();
			
			if((m == -1) & (f == -1) & (r == -1)){
				break;
			}
			
			if((m == -1) | (f == -1)){//??????F
				sb.append("F");
				sb.append("\n");
			}else if((m + f)>=80){//??????A
				sb.append("A");
				sb.append("\n");
			}else if((m + f)>=65){//??????B
				sb.append("B");
				sb.append("\n");
			}else if((m + f)>=50){//??????C
				sb.append("C");
				sb.append("\n");
			}else if((m + f)>=30){//??????D ???r???50??\????????´????????????C
				if(r >= 50){//??????C
				sb.append("C");
				sb.append("\n");
				}else{//??????D
					sb.append("D");
					sb.append("\n");
				}
			}else if((m+f)<30){//??????F
				sb.append("F");
				sb.append("\n");
			}
		}
		System.out.print(new String(sb));
		
	}
}
/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