import java.util.Scanner;

public class Main {
	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		while(sc.hasNext()) {
		int m = sc.nextInt();
		int f = sc.nextInt();
		int r = sc.nextInt();
		int p = m + f;
		
		if(m == -1 && f == -1)
				break;
		
		if(m == -1 || f == -1)
			System.out.println("F");
		else if(p >= 80)
			System.out.println("A");
		else if(p >= 65)
			System.out.println("B");
		else if(p >= 50)
			System.out.println("C");
		else if(p >= 30)
			if(r < 50)
				System.out.println("D");
			else
				System.out.println("C");
		else
			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(sc.hasNext()) {
		int m = sc.nextInt();
		int f = sc.nextInt();
		int r = sc.nextInt();
		int p = m + f;
		
		if(m == -1 && f == -1)
				break;
		
		if(m == -1 || f == -1)
			System.out.println("F");
		else if(p >= 80)
			System.out.println("A");
		else if(p >= 65)
			System.out.println("B");
		else if(p >= 50)
			System.out.println("C");
		else if(p >= 30)
			if(r < 50)
				System.out.println("D");
			else
				System.out.println("C");
		else
			System.out.println("F");
		
		}
	}
} Main public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		while(sc.hasNext()) {
		int m = sc.nextInt();
		int f = sc.nextInt();
		int r = sc.nextInt();
		int p = m + f;
		
		if(m == -1 && f == -1)
				break;
		
		if(m == -1 || f == -1)
			System.out.println("F");
		else if(p >= 80)
			System.out.println("A");
		else if(p >= 65)
			System.out.println("B");
		else if(p >= 50)
			System.out.println("C");
		else if(p >= 30)
			if(r < 50)
				System.out.println("D");
			else
				System.out.println("C");
		else
			System.out.println("F");
		
		}
	} main {
		Scanner sc = new Scanner(System.in);
		while(sc.hasNext()) {
		int m = sc.nextInt();
		int f = sc.nextInt();
		int r = sc.nextInt();
		int p = m + f;
		
		if(m == -1 && f == -1)
				break;
		
		if(m == -1 || f == -1)
			System.out.println("F");
		else if(p >= 80)
			System.out.println("A");
		else if(p >= 65)
			System.out.println("B");
		else if(p >= 50)
			System.out.println("C");
		else if(p >= 30)
			if(r < 50)
				System.out.println("D");
			else
				System.out.println("C");
		else
			System.out.println("F");
		
		}
	} Scanner sc = new Scanner(System.in); sc new Scanner(System.in) while(sc.hasNext()) {
		int m = sc.nextInt();
		int f = sc.nextInt();
		int r = sc.nextInt();
		int p = m + f;
		
		if(m == -1 && f == -1)
				break;
		
		if(m == -1 || f == -1)
			System.out.println("F");
		else if(p >= 80)
			System.out.println("A");
		else if(p >= 65)
			System.out.println("B");
		else if(p >= 50)
			System.out.println("C");
		else if(p >= 30)
			if(r < 50)
				System.out.println("D");
			else
				System.out.println("C");
		else
			System.out.println("F");
		
		} sc.hasNext() sc.hasNext sc {
		int m = sc.nextInt();
		int f = sc.nextInt();
		int r = sc.nextInt();
		int p = m + f;
		
		if(m == -1 && f == -1)
				break;
		
		if(m == -1 || f == -1)
			System.out.println("F");
		else if(p >= 80)
			System.out.println("A");
		else if(p >= 65)
			System.out.println("B");
		else if(p >= 50)
			System.out.println("C");
		else if(p >= 30)
			if(r < 50)
				System.out.println("D");
			else
				System.out.println("C");
		else
			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 int p = m + f; p m + f m f if(m == -1 && f == -1)
				break; m == -1 && f == -1 m == -1 m -1 1 f == -1 f -1 1 break; if(m == -1 || f == -1)
			System.out.println("F");
		else if(p >= 80)
			System.out.println("A");
		else if(p >= 65)
			System.out.println("B");
		else if(p >= 50)
			System.out.println("C");
		else if(p >= 30)
			if(r < 50)
				System.out.println("D");
			else
				System.out.println("C");
		else
			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 System.out System System.out "F" if(p >= 80)
			System.out.println("A");
		else if(p >= 65)
			System.out.println("B");
		else if(p >= 50)
			System.out.println("C");
		else if(p >= 30)
			if(r < 50)
				System.out.println("D");
			else
				System.out.println("C");
		else
			System.out.println("F"); p >= 80 p 80 System.out.println("A") System.out.println System.out System System.out "A" if(p >= 65)
			System.out.println("B");
		else if(p >= 50)
			System.out.println("C");
		else if(p >= 30)
			if(r < 50)
				System.out.println("D");
			else
				System.out.println("C");
		else
			System.out.println("F"); p >= 65 p 65 System.out.println("B") System.out.println System.out System System.out "B" if(p >= 50)
			System.out.println("C");
		else if(p >= 30)
			if(r < 50)
				System.out.println("D");
			else
				System.out.println("C");
		else
			System.out.println("F"); p >= 50 p 50 System.out.println("C") System.out.println System.out System System.out "C" if(p >= 30)
			if(r < 50)
				System.out.println("D");
			else
				System.out.println("C");
		else
			System.out.println("F"); p >= 30 p 30 if(r < 50)
				System.out.println("D");
			else
				System.out.println("C"); r < 50 r 50 System.out.println("D") System.out.println System.out System System.out "D" System.out.println("C") System.out.println System.out System System.out "C" 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(sc.hasNext()) {
		int m = sc.nextInt();
		int f = sc.nextInt();
		int r = sc.nextInt();
		int p = m + f;
		
		if(m == -1 && f == -1)
				break;
		
		if(m == -1 || f == -1)
			System.out.println("F");
		else if(p >= 80)
			System.out.println("A");
		else if(p >= 65)
			System.out.println("B");
		else if(p >= 50)
			System.out.println("C");
		else if(p >= 30)
			if(r < 50)
				System.out.println("D");
			else
				System.out.println("C");
		else
			System.out.println("F");
		
		}
	}
} public class Main {
	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		while(sc.hasNext()) {
		int m = sc.nextInt();
		int f = sc.nextInt();
		int r = sc.nextInt();
		int p = m + f;
		
		if(m == -1 && f == -1)
				break;
		
		if(m == -1 || f == -1)
			System.out.println("F");
		else if(p >= 80)
			System.out.println("A");
		else if(p >= 65)
			System.out.println("B");
		else if(p >= 50)
			System.out.println("C");
		else if(p >= 30)
			if(r < 50)
				System.out.println("D");
			else
				System.out.println("C");
		else
			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(sc.hasNext()) {
		int m = sc.nextInt();
		int f = sc.nextInt();
		int r = sc.nextInt();
		int p = m + f;
		
		if(m == -1 && f == -1)
				break;
		
		if(m == -1 || f == -1)
			System.out.println("F");
		else if(p >= 80)
			System.out.println("A");
		else if(p >= 65)
			System.out.println("B");
		else if(p >= 50)
			System.out.println("C");
		else if(p >= 30)
			if(r < 50)
				System.out.println("D");
			else
				System.out.println("C");
		else
			System.out.println("F");
		
		}
	}
}

