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");
            } else if(m+f >= 80){
                System.out.println("A");
            } else if(m+f >= 65){
                System.out.println("B");
            } else if(m+f >= 50 || r >= 50){
                System.out.println("C");
            } else if(m+f >= 30){
                System.out.println("D");
            } 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(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");
            } else if(m+f >= 80){
                System.out.println("A");
            } else if(m+f >= 65){
                System.out.println("B");
            } else if(m+f >= 50 || r >= 50){
                System.out.println("C");
            } else if(m+f >= 30){
                System.out.println("D");
            } else {
                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;

            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 || r >= 50){
                System.out.println("C");
            } else if(m+f >= 30){
                System.out.println("D");
            } else {
                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;

            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 || r >= 50){
                System.out.println("C");
            } else if(m+f >= 30){
                System.out.println("D");
            } else {
                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;

            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 || r >= 50){
                System.out.println("C");
            } else if(m+f >= 30){
                System.out.println("D");
            } else {
                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;

            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 || r >= 50){
                System.out.println("C");
            } else if(m+f >= 30){
                System.out.println("D");
            } 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 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; 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 || r >= 50){
                System.out.println("C");
            } else if(m+f >= 30){
                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 || r >= 50){
                System.out.println("C");
            } else if(m+f >= 30){
                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 || r >= 50){
                System.out.println("C");
            } else if(m+f >= 30){
                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 || r >= 50){
                System.out.println("C");
            } else if(m+f >= 30){
                System.out.println("D");
            } else {
                System.out.println("F");
            } m+f >= 50 || r >= 50 m+f >= 50 m+f m f 50 r >= 50 r 50 {
                System.out.println("C");
            } System.out.println("C") System.out.println System.out System System.out "C" if(m+f >= 30){
                System.out.println("D");
            } else {
                System.out.println("F");
            } m+f >= 30 m+f m f 30 {
                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){
            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");
            } else if(m+f >= 80){
                System.out.println("A");
            } else if(m+f >= 65){
                System.out.println("B");
            } else if(m+f >= 50 || r >= 50){
                System.out.println("C");
            } else if(m+f >= 30){
                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){
            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");
            } else if(m+f >= 80){
                System.out.println("A");
            } else if(m+f >= 65){
                System.out.println("B");
            } else if(m+f >= 50 || r >= 50){
                System.out.println("C");
            } else if(m+f >= 30){
                System.out.println("D");
            } 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(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");
            } else if(m+f >= 80){
                System.out.println("A");
            } else if(m+f >= 65){
                System.out.println("B");
            } else if(m+f >= 50 || r >= 50){
                System.out.println("C");
            } else if(m+f >= 30){
                System.out.println("D");
            } else {
                System.out.println("F");
            }
        }
    }
}


