import java.util.*;

class Main{
	public static void main(String[] args){
		Scanner sc = new Scanner(System.in);
		
		while(true){
			String[] line = sc.nextLine().split(" ");
			int m = Integer.parseInt(line[0]);
			int f = Integer.parseInt(line[1]);
			int r = Integer.parseInt(line[2]);
		//finish	
			if(m==-1 && f==-1 && r==-1){
				break;
			}
			int sum = m+f;
		//out
			if(m==-1 || f==-1){
				System.out.println("F");
			}else if(sum>=80){
				System.out.println("A");
			}else if(sum>=65 && sum<80){
				System.out.println("B");
			}else if(sum>=50 && sum<65){
				System.out.println("C");
			}else if(sum>=30 && sum<50){
				if(r>=50){
					System.out.println("C");
				}else{
					System.out.println("D");
				}
			}else if(sum<30){
				System.out.println("F");
			}
		}
	}
} import java.util.*; util.* java class Main{
	public static void main(String[] args){
		Scanner sc = new Scanner(System.in);
		
		while(true){
			String[] line = sc.nextLine().split(" ");
			int m = Integer.parseInt(line[0]);
			int f = Integer.parseInt(line[1]);
			int r = Integer.parseInt(line[2]);
		//finish	
			if(m==-1 && f==-1 && r==-1){
				break;
			}
			int sum = m+f;
		//out
			if(m==-1 || f==-1){
				System.out.println("F");
			}else if(sum>=80){
				System.out.println("A");
			}else if(sum>=65 && sum<80){
				System.out.println("B");
			}else if(sum>=50 && sum<65){
				System.out.println("C");
			}else if(sum>=30 && sum<50){
				if(r>=50){
					System.out.println("C");
				}else{
					System.out.println("D");
				}
			}else if(sum<30){
				System.out.println("F");
			}
		}
	}
} Main public static void main(String[] args){
		Scanner sc = new Scanner(System.in);
		
		while(true){
			String[] line = sc.nextLine().split(" ");
			int m = Integer.parseInt(line[0]);
			int f = Integer.parseInt(line[1]);
			int r = Integer.parseInt(line[2]);
		//finish	
			if(m==-1 && f==-1 && r==-1){
				break;
			}
			int sum = m+f;
		//out
			if(m==-1 || f==-1){
				System.out.println("F");
			}else if(sum>=80){
				System.out.println("A");
			}else if(sum>=65 && sum<80){
				System.out.println("B");
			}else if(sum>=50 && sum<65){
				System.out.println("C");
			}else if(sum>=30 && sum<50){
				if(r>=50){
					System.out.println("C");
				}else{
					System.out.println("D");
				}
			}else if(sum<30){
				System.out.println("F");
			}
		}
	} main {
		Scanner sc = new Scanner(System.in);
		
		while(true){
			String[] line = sc.nextLine().split(" ");
			int m = Integer.parseInt(line[0]);
			int f = Integer.parseInt(line[1]);
			int r = Integer.parseInt(line[2]);
		//finish	
			if(m==-1 && f==-1 && r==-1){
				break;
			}
			int sum = m+f;
		//out
			if(m==-1 || f==-1){
				System.out.println("F");
			}else if(sum>=80){
				System.out.println("A");
			}else if(sum>=65 && sum<80){
				System.out.println("B");
			}else if(sum>=50 && sum<65){
				System.out.println("C");
			}else if(sum>=30 && sum<50){
				if(r>=50){
					System.out.println("C");
				}else{
					System.out.println("D");
				}
			}else if(sum<30){
				System.out.println("F");
			}
		}
	} Scanner sc = new Scanner(System.in); sc new Scanner(System.in) while(true){
			String[] line = sc.nextLine().split(" ");
			int m = Integer.parseInt(line[0]);
			int f = Integer.parseInt(line[1]);
			int r = Integer.parseInt(line[2]);
		//finish	
			if(m==-1 && f==-1 && r==-1){
				break;
			}
			int sum = m+f;
		//out
			if(m==-1 || f==-1){
				System.out.println("F");
			}else if(sum>=80){
				System.out.println("A");
			}else if(sum>=65 && sum<80){
				System.out.println("B");
			}else if(sum>=50 && sum<65){
				System.out.println("C");
			}else if(sum>=30 && sum<50){
				if(r>=50){
					System.out.println("C");
				}else{
					System.out.println("D");
				}
			}else if(sum<30){
				System.out.println("F");
			}
		} true {
			String[] line = sc.nextLine().split(" ");
			int m = Integer.parseInt(line[0]);
			int f = Integer.parseInt(line[1]);
			int r = Integer.parseInt(line[2]);
		//finish	
			if(m==-1 && f==-1 && r==-1){
				break;
			}
			int sum = m+f;
		//out
			if(m==-1 || f==-1){
				System.out.println("F");
			}else if(sum>=80){
				System.out.println("A");
			}else if(sum>=65 && sum<80){
				System.out.println("B");
			}else if(sum>=50 && sum<65){
				System.out.println("C");
			}else if(sum>=30 && sum<50){
				if(r>=50){
					System.out.println("C");
				}else{
					System.out.println("D");
				}
			}else if(sum<30){
				System.out.println("F");
			}
		} String[] line = sc.nextLine().split(" "); line sc.nextLine().split(" ") sc.nextLine().split sc.nextLine() sc.nextLine sc " " int m = Integer.parseInt(line[0]); m Integer.parseInt(line[0]) Integer.parseInt Integer line[0] line 0 int f = Integer.parseInt(line[1]); f Integer.parseInt(line[1]) Integer.parseInt Integer line[1] line 1 int r = Integer.parseInt(line[2]); r Integer.parseInt(line[2]) Integer.parseInt Integer line[2] line 2 if(m==-1 && f==-1 && r==-1){
				break;
			} m==-1 && f==-1 && r==-1 m==-1 && f==-1 && r==-1 m==-1 m -1 1 f==-1 f -1 1 r==-1 r -1 1 {
				break;
			} break; int sum = m+f; sum m+f m f if(m==-1 || f==-1){
				System.out.println("F");
			}else if(sum>=80){
				System.out.println("A");
			}else if(sum>=65 && sum<80){
				System.out.println("B");
			}else if(sum>=50 && sum<65){
				System.out.println("C");
			}else if(sum>=30 && sum<50){
				if(r>=50){
					System.out.println("C");
				}else{
					System.out.println("D");
				}
			}else if(sum<30){
				System.out.println("F");
			} m==-1 || f==-1 m==-1 m -1 1 f==-1 f -1 1 {
				System.out.println("F");
			} System.out.println("F") System.out.println System.out System System.out "F" if(sum>=80){
				System.out.println("A");
			}else if(sum>=65 && sum<80){
				System.out.println("B");
			}else if(sum>=50 && sum<65){
				System.out.println("C");
			}else if(sum>=30 && sum<50){
				if(r>=50){
					System.out.println("C");
				}else{
					System.out.println("D");
				}
			}else if(sum<30){
				System.out.println("F");
			} sum>=80 sum 80 {
				System.out.println("A");
			} System.out.println("A") System.out.println System.out System System.out "A" if(sum>=65 && sum<80){
				System.out.println("B");
			}else if(sum>=50 && sum<65){
				System.out.println("C");
			}else if(sum>=30 && sum<50){
				if(r>=50){
					System.out.println("C");
				}else{
					System.out.println("D");
				}
			}else if(sum<30){
				System.out.println("F");
			} sum>=65 && sum<80 sum>=65 sum 65 sum<80 sum 80 {
				System.out.println("B");
			} System.out.println("B") System.out.println System.out System System.out "B" if(sum>=50 && sum<65){
				System.out.println("C");
			}else if(sum>=30 && sum<50){
				if(r>=50){
					System.out.println("C");
				}else{
					System.out.println("D");
				}
			}else if(sum<30){
				System.out.println("F");
			} sum>=50 && sum<65 sum>=50 sum 50 sum<65 sum 65 {
				System.out.println("C");
			} System.out.println("C") System.out.println System.out System System.out "C" if(sum>=30 && sum<50){
				if(r>=50){
					System.out.println("C");
				}else{
					System.out.println("D");
				}
			}else if(sum<30){
				System.out.println("F");
			} sum>=30 && sum<50 sum>=30 sum 30 sum<50 sum 50 {
				if(r>=50){
					System.out.println("C");
				}else{
					System.out.println("D");
				}
			} if(r>=50){
					System.out.println("C");
				}else{
					System.out.println("D");
				} r>=50 r 50 {
					System.out.println("C");
				} System.out.println("C") System.out.println System.out System System.out "C" {
					System.out.println("D");
				} System.out.println("D") System.out.println System.out System System.out "D" if(sum<30){
				System.out.println("F");
			} sum<30 sum 30 {
				System.out.println("F");
			} System.out.println("F") System.out.println System.out System System.out "F" String[] args args
import java.util.*;

class Main{
	public static void main(String[] args){
		Scanner sc = new Scanner(System.in);
		
		while(true){
			String[] line = sc.nextLine().split(" ");
			int m = Integer.parseInt(line[0]);
			int f = Integer.parseInt(line[1]);
			int r = Integer.parseInt(line[2]);
		//finish	
			if(m==-1 && f==-1 && r==-1){
				break;
			}
			int sum = m+f;
		//out
			if(m==-1 || f==-1){
				System.out.println("F");
			}else if(sum>=80){
				System.out.println("A");
			}else if(sum>=65 && sum<80){
				System.out.println("B");
			}else if(sum>=50 && sum<65){
				System.out.println("C");
			}else if(sum>=30 && sum<50){
				if(r>=50){
					System.out.println("C");
				}else{
					System.out.println("D");
				}
			}else if(sum<30){
				System.out.println("F");
			}
		}
	}
}