import java.util.Scanner; 
public class Main { 
	public static void main(String... args) { 
		Scanner sc = new Scanner(System.in); 
		while(true) { 
			
			int m = sc.nextInt(); 
			int f = sc.nextInt(); 
			int r = sc.nextInt(); 
			
			if(m == -1 && f == -1 && r == -1) break; 
			
			if(m == -1 || f == -1) { 
				System.out.println('F'); 
				continue; 
			} 
			if(m + f >= 80) { 
				System.out.println('A'); 
				continue; 
			} 
			if(m + f >= 65) { 
				System.out.println('B'); 
				continue; 
			} 
			if(m + f >= 50) { 
				System.out.println('C'); 
				continue; 
			} 
			if(m + f >= 30) { 
				if(r >= 50) System.out.println('C'); 
				else System.out.println('D'); 
				continue; 
			}
			
			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