

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;
			}

			int sum = m + f;


			if(m == -1 || f == -1){
				System.out.println('F');
			}
			else if(80 <= sum){
				System.out.println('A');
			}
			else if(65 <= sum && sum < 80){
				System.out.println('B');
			}
			else if(50 <= sum && sum < 65){
				System.out.println('C');
			}
			else if(30 <= sum && sum < 50){

				if(r >= 50){
					System.out.println('C');
				}
				else{
					System.out.println('D');
				}
			}
			if(30 > sum){
				System.out.println('F');


			}



		}

	}

} import java.util.Scanner; Scanner java.util 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;
			}

			int sum = m + f;


			if(m == -1 || f == -1){
				System.out.println('F');
			}
			else if(80 <= sum){
				System.out.println('A');
			}
			else if(65 <= sum && sum < 80){
				System.out.println('B');
			}
			else if(50 <= sum && sum < 65){
				System.out.println('C');
			}
			else if(30 <= sum && sum < 50){

				if(r >= 50){
					System.out.println('C');
				}
				else{
					System.out.println('D');
				}
			}
			if(30 > sum){
				System.out.println('F');


			}



		}

	}

} 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;
			}

			int sum = m + f;


			if(m == -1 || f == -1){
				System.out.println('F');
			}
			else if(80 <= sum){
				System.out.println('A');
			}
			else if(65 <= sum && sum < 80){
				System.out.println('B');
			}
			else if(50 <= sum && sum < 65){
				System.out.println('C');
			}
			else if(30 <= sum && sum < 50){

				if(r >= 50){
					System.out.println('C');
				}
				else{
					System.out.println('D');
				}
			}
			if(30 > sum){
				System.out.println('F');


			}



		}

	} main {


		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;
			}

			int sum = m + f;


			if(m == -1 || f == -1){
				System.out.println('F');
			}
			else if(80 <= sum){
				System.out.println('A');
			}
			else if(65 <= sum && sum < 80){
				System.out.println('B');
			}
			else if(50 <= sum && sum < 65){
				System.out.println('C');
			}
			else if(30 <= sum && sum < 50){

				if(r >= 50){
					System.out.println('C');
				}
				else{
					System.out.println('D');
				}
			}
			if(30 > sum){
				System.out.println('F');


			}



		}

	} Scanner sc = new Scanner(System.in); 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;
			}

			int sum = m + f;


			if(m == -1 || f == -1){
				System.out.println('F');
			}
			else if(80 <= sum){
				System.out.println('A');
			}
			else if(65 <= sum && sum < 80){
				System.out.println('B');
			}
			else if(50 <= sum && sum < 65){
				System.out.println('C');
			}
			else if(30 <= sum && sum < 50){

				if(r >= 50){
					System.out.println('C');
				}
				else{
					System.out.println('D');
				}
			}
			if(30 > sum){
				System.out.println('F');


			}



		} true {
			int m = sc.nextInt();
			int f = sc.nextInt();
			int r = sc.nextInt();


			if(m == -1 && f == -1 && r == -1){
				break;
			}

			int sum = m + f;


			if(m == -1 || f == -1){
				System.out.println('F');
			}
			else if(80 <= sum){
				System.out.println('A');
			}
			else if(65 <= sum && sum < 80){
				System.out.println('B');
			}
			else if(50 <= sum && sum < 65){
				System.out.println('C');
			}
			else if(30 <= sum && sum < 50){

				if(r >= 50){
					System.out.println('C');
				}
				else{
					System.out.println('D');
				}
			}
			if(30 > sum){
				System.out.println('F');


			}



		} int m = sc.nextInt(); m sc.nextInt() sc.nextInt sc int f = sc.nextInt(); f sc.nextInt() sc.nextInt sc int r = sc.nextInt(); r sc.nextInt() sc.nextInt sc 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(80 <= sum){
				System.out.println('A');
			}
			else if(65 <= sum && sum < 80){
				System.out.println('B');
			}
			else if(50 <= sum && sum < 65){
				System.out.println('C');
			}
			else if(30 <= sum && sum < 50){

				if(r >= 50){
					System.out.println('C');
				}
				else{
					System.out.println('D');
				}
			} 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(80 <= sum){
				System.out.println('A');
			}
			else if(65 <= sum && sum < 80){
				System.out.println('B');
			}
			else if(50 <= sum && sum < 65){
				System.out.println('C');
			}
			else if(30 <= sum && sum < 50){

				if(r >= 50){
					System.out.println('C');
				}
				else{
					System.out.println('D');
				}
			} 80 <= sum 80 sum {
				System.out.println('A');
			} System.out.println('A') System.out.println System.out System System.out 'A' if(65 <= sum && sum < 80){
				System.out.println('B');
			}
			else if(50 <= sum && sum < 65){
				System.out.println('C');
			}
			else if(30 <= sum && sum < 50){

				if(r >= 50){
					System.out.println('C');
				}
				else{
					System.out.println('D');
				}
			} 65 <= sum && sum < 80 65 <= sum 65 sum sum < 80 sum 80 {
				System.out.println('B');
			} System.out.println('B') System.out.println System.out System System.out 'B' if(50 <= sum && sum < 65){
				System.out.println('C');
			}
			else if(30 <= sum && sum < 50){

				if(r >= 50){
					System.out.println('C');
				}
				else{
					System.out.println('D');
				}
			} 50 <= sum && sum < 65 50 <= sum 50 sum sum < 65 sum 65 {
				System.out.println('C');
			} System.out.println('C') System.out.println System.out System System.out 'C' if(30 <= sum && sum < 50){

				if(r >= 50){
					System.out.println('C');
				}
				else{
					System.out.println('D');
				}
			} 30 <= sum && sum < 50 30 <= sum 30 sum 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(30 > sum){
				System.out.println('F');


			} 30 > sum 30 sum {
				System.out.println('F');


			} System.out.println('F') System.out.println System.out System System.out 'F' String[] args args 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;
			}

			int sum = m + f;


			if(m == -1 || f == -1){
				System.out.println('F');
			}
			else if(80 <= sum){
				System.out.println('A');
			}
			else if(65 <= sum && sum < 80){
				System.out.println('B');
			}
			else if(50 <= sum && sum < 65){
				System.out.println('C');
			}
			else if(30 <= sum && sum < 50){

				if(r >= 50){
					System.out.println('C');
				}
				else{
					System.out.println('D');
				}
			}
			if(30 > sum){
				System.out.println('F');


			}



		}

	}

} 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;
			}

			int sum = m + f;


			if(m == -1 || f == -1){
				System.out.println('F');
			}
			else if(80 <= sum){
				System.out.println('A');
			}
			else if(65 <= sum && sum < 80){
				System.out.println('B');
			}
			else if(50 <= sum && sum < 65){
				System.out.println('C');
			}
			else if(30 <= sum && sum < 50){

				if(r >= 50){
					System.out.println('C');
				}
				else{
					System.out.println('D');
				}
			}
			if(30 > sum){
				System.out.println('F');


			}



		}

	}

} Main


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;
			}

			int sum = m + f;


			if(m == -1 || f == -1){
				System.out.println('F');
			}
			else if(80 <= sum){
				System.out.println('A');
			}
			else if(65 <= sum && sum < 80){
				System.out.println('B');
			}
			else if(50 <= sum && sum < 65){
				System.out.println('C');
			}
			else if(30 <= sum && sum < 50){

				if(r >= 50){
					System.out.println('C');
				}
				else{
					System.out.println('D');
				}
			}
			if(30 > sum){
				System.out.println('F');


			}



		}

	}

}