import java.util.Arrays;
import java.util.HashMap;
import java.util.Map;
import java.util.Scanner;

public class Main {

	public static void main(String args[]) {
		Scanner sc = new Scanner(System.in);

		while (true) {
			final int m = sc.nextInt();
			final int f = sc.nextInt();
			final int r = sc.nextInt();
			
			if(m == -1 && f == -1 && r == -1){
				break;
			}
			
			if (m == -1 || f == -1) {
				System.out.println("F");
			} else if (m + f >= 80) {
				System.out.println("A");
			} else if (m + f >= 65) {
				System.out.println("B");
			} else if (m + f >= 50) {
				System.out.println("C");
			} else if (m + f >= 30) {
				if (r >= 50) {
					System.out.println("C");
				} else {
					System.out.println("D");
				}
			} else {
				System.out.println("F");
			}
		}
	}

} import java.util.Arrays; Arrays java.util import java.util.HashMap; HashMap java.util import java.util.Map; Map java.util import java.util.Scanner; Scanner java.util public class Main {

	public static void main(String args[]) {
		Scanner sc = new Scanner(System.in);

		while (true) {
			final int m = sc.nextInt();
			final int f = sc.nextInt();
			final int r = sc.nextInt();
			
			if(m == -1 && f == -1 && r == -1){
				break;
			}
			
			if (m == -1 || f == -1) {
				System.out.println("F");
			} else if (m + f >= 80) {
				System.out.println("A");
			} else if (m + f >= 65) {
				System.out.println("B");
			} else if (m + f >= 50) {
				System.out.println("C");
			} else if (m + f >= 30) {
				if (r >= 50) {
					System.out.println("C");
				} else {
					System.out.println("D");
				}
			} else {
				System.out.println("F");
			}
		}
	}

} Main public static void main(String args[]) {
		Scanner sc = new Scanner(System.in);

		while (true) {
			final int m = sc.nextInt();
			final int f = sc.nextInt();
			final int r = sc.nextInt();
			
			if(m == -1 && f == -1 && r == -1){
				break;
			}
			
			if (m == -1 || f == -1) {
				System.out.println("F");
			} else if (m + f >= 80) {
				System.out.println("A");
			} else if (m + f >= 65) {
				System.out.println("B");
			} else if (m + f >= 50) {
				System.out.println("C");
			} else if (m + f >= 30) {
				if (r >= 50) {
					System.out.println("C");
				} else {
					System.out.println("D");
				}
			} else {
				System.out.println("F");
			}
		}
	} main {
		Scanner sc = new Scanner(System.in);

		while (true) {
			final int m = sc.nextInt();
			final int f = sc.nextInt();
			final int r = sc.nextInt();
			
			if(m == -1 && f == -1 && r == -1){
				break;
			}
			
			if (m == -1 || f == -1) {
				System.out.println("F");
			} else if (m + f >= 80) {
				System.out.println("A");
			} else if (m + f >= 65) {
				System.out.println("B");
			} else if (m + f >= 50) {
				System.out.println("C");
			} else if (m + f >= 30) {
				if (r >= 50) {
					System.out.println("C");
				} else {
					System.out.println("D");
				}
			} else {
				System.out.println("F");
			}
		}
	} Scanner sc = new Scanner(System.in); sc new Scanner(System.in) while (true) {
			final int m = sc.nextInt();
			final int f = sc.nextInt();
			final int r = sc.nextInt();
			
			if(m == -1 && f == -1 && r == -1){
				break;
			}
			
			if (m == -1 || f == -1) {
				System.out.println("F");
			} else if (m + f >= 80) {
				System.out.println("A");
			} else if (m + f >= 65) {
				System.out.println("B");
			} else if (m + f >= 50) {
				System.out.println("C");
			} else if (m + f >= 30) {
				if (r >= 50) {
					System.out.println("C");
				} else {
					System.out.println("D");
				}
			} else {
				System.out.println("F");
			}
		} true {
			final int m = sc.nextInt();
			final int f = sc.nextInt();
			final int r = sc.nextInt();
			
			if(m == -1 && f == -1 && r == -1){
				break;
			}
			
			if (m == -1 || f == -1) {
				System.out.println("F");
			} else if (m + f >= 80) {
				System.out.println("A");
			} else if (m + f >= 65) {
				System.out.println("B");
			} else if (m + f >= 50) {
				System.out.println("C");
			} else if (m + f >= 30) {
				if (r >= 50) {
					System.out.println("C");
				} else {
					System.out.println("D");
				}
			} else {
				System.out.println("F");
			}
		} final int m = sc.nextInt(); m sc.nextInt() sc.nextInt sc final int f = sc.nextInt(); f sc.nextInt() sc.nextInt sc final 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; if (m == -1 || f == -1) {
				System.out.println("F");
			} else if (m + f >= 80) {
				System.out.println("A");
			} else if (m + f >= 65) {
				System.out.println("B");
			} else if (m + f >= 50) {
				System.out.println("C");
			} else if (m + f >= 30) {
				if (r >= 50) {
					System.out.println("C");
				} else {
					System.out.println("D");
				}
			} 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("F") System.out.println System.out System System.out "F" if (m + f >= 80) {
				System.out.println("A");
			} else if (m + f >= 65) {
				System.out.println("B");
			} else if (m + f >= 50) {
				System.out.println("C");
			} else if (m + f >= 30) {
				if (r >= 50) {
					System.out.println("C");
				} else {
					System.out.println("D");
				}
			} else {
				System.out.println("F");
			} m + f >= 80 m + f m f 80 {
				System.out.println("A");
			} System.out.println("A") System.out.println System.out System System.out "A" if (m + f >= 65) {
				System.out.println("B");
			} else if (m + f >= 50) {
				System.out.println("C");
			} else if (m + f >= 30) {
				if (r >= 50) {
					System.out.println("C");
				} else {
					System.out.println("D");
				}
			} else {
				System.out.println("F");
			} m + f >= 65 m + f m f 65 {
				System.out.println("B");
			} System.out.println("B") System.out.println System.out System System.out "B" if (m + f >= 50) {
				System.out.println("C");
			} else if (m + f >= 30) {
				if (r >= 50) {
					System.out.println("C");
				} else {
					System.out.println("D");
				}
			} else {
				System.out.println("F");
			} m + f >= 50 m + f m f 50 {
				System.out.println("C");
			} System.out.println("C") System.out.println System.out System System.out "C" if (m + f >= 30) {
				if (r >= 50) {
					System.out.println("C");
				} else {
					System.out.println("D");
				}
			} else {
				System.out.println("F");
			} m + f >= 30 m + f m f 30 {
				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" {
				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) {
			final int m = sc.nextInt();
			final int f = sc.nextInt();
			final int r = sc.nextInt();
			
			if(m == -1 && f == -1 && r == -1){
				break;
			}
			
			if (m == -1 || f == -1) {
				System.out.println("F");
			} else if (m + f >= 80) {
				System.out.println("A");
			} else if (m + f >= 65) {
				System.out.println("B");
			} else if (m + f >= 50) {
				System.out.println("C");
			} else if (m + f >= 30) {
				if (r >= 50) {
					System.out.println("C");
				} else {
					System.out.println("D");
				}
			} else {
				System.out.println("F");
			}
		}
	}

} public class Main {

	public static void main(String args[]) {
		Scanner sc = new Scanner(System.in);

		while (true) {
			final int m = sc.nextInt();
			final int f = sc.nextInt();
			final int r = sc.nextInt();
			
			if(m == -1 && f == -1 && r == -1){
				break;
			}
			
			if (m == -1 || f == -1) {
				System.out.println("F");
			} else if (m + f >= 80) {
				System.out.println("A");
			} else if (m + f >= 65) {
				System.out.println("B");
			} else if (m + f >= 50) {
				System.out.println("C");
			} else if (m + f >= 30) {
				if (r >= 50) {
					System.out.println("C");
				} else {
					System.out.println("D");
				}
			} else {
				System.out.println("F");
			}
		}
	}

} Main
import java.util.Arrays;
import java.util.HashMap;
import java.util.Map;
import java.util.Scanner;

public class Main {

	public static void main(String args[]) {
		Scanner sc = new Scanner(System.in);

		while (true) {
			final int m = sc.nextInt();
			final int f = sc.nextInt();
			final int r = sc.nextInt();
			
			if(m == -1 && f == -1 && r == -1){
				break;
			}
			
			if (m == -1 || f == -1) {
				System.out.println("F");
			} else if (m + f >= 80) {
				System.out.println("A");
			} else if (m + f >= 65) {
				System.out.println("B");
			} else if (m + f >= 50) {
				System.out.println("C");
			} else if (m + f >= 30) {
				if (r >= 50) {
					System.out.println("C");
				} else {
					System.out.println("D");
				}
			} else {
				System.out.println("F");
			}
		}
	}

}