import java.util.Scanner;

public class Main {
	public static void main(String [] args){
        Scanner scan = new Scanner(System.in);
        
        while(true){
            int m = scan.nextInt();
            int f = scan.nextInt();
            int r = scan.nextInt();
            
            if(m < 0 && f < 0 && r < 0){
                break;
            }
            
            if(m < 0 || f < 0){
                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.Scanner; Scanner java.util public class Main {
	public static void main(String [] args){
        Scanner scan = new Scanner(System.in);
        
        while(true){
            int m = scan.nextInt();
            int f = scan.nextInt();
            int r = scan.nextInt();
            
            if(m < 0 && f < 0 && r < 0){
                break;
            }
            
            if(m < 0 || f < 0){
                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 scan = new Scanner(System.in);
        
        while(true){
            int m = scan.nextInt();
            int f = scan.nextInt();
            int r = scan.nextInt();
            
            if(m < 0 && f < 0 && r < 0){
                break;
            }
            
            if(m < 0 || f < 0){
                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 scan = new Scanner(System.in);
        
        while(true){
            int m = scan.nextInt();
            int f = scan.nextInt();
            int r = scan.nextInt();
            
            if(m < 0 && f < 0 && r < 0){
                break;
            }
            
            if(m < 0 || f < 0){
                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 scan = new Scanner(System.in); scan new Scanner(System.in) while(true){
            int m = scan.nextInt();
            int f = scan.nextInt();
            int r = scan.nextInt();
            
            if(m < 0 && f < 0 && r < 0){
                break;
            }
            
            if(m < 0 || f < 0){
                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 {
            int m = scan.nextInt();
            int f = scan.nextInt();
            int r = scan.nextInt();
            
            if(m < 0 && f < 0 && r < 0){
                break;
            }
            
            if(m < 0 || f < 0){
                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');
            }
        } int m = scan.nextInt(); m scan.nextInt() scan.nextInt scan int f = scan.nextInt(); f scan.nextInt() scan.nextInt scan int r = scan.nextInt(); r scan.nextInt() scan.nextInt scan if(m < 0 && f < 0 && r < 0){
                break;
            } m < 0 && f < 0 && r < 0 m < 0 && f < 0 && r < 0 m < 0 m 0 f < 0 f 0 r < 0 r 0 {
                break;
            } break; if(m < 0 || f < 0){
                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 < 0 || f < 0 m < 0 m 0 f < 0 f 0 {
                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 scan = new Scanner(System.in);
        
        while(true){
            int m = scan.nextInt();
            int f = scan.nextInt();
            int r = scan.nextInt();
            
            if(m < 0 && f < 0 && r < 0){
                break;
            }
            
            if(m < 0 || f < 0){
                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 scan = new Scanner(System.in);
        
        while(true){
            int m = scan.nextInt();
            int f = scan.nextInt();
            int r = scan.nextInt();
            
            if(m < 0 && f < 0 && r < 0){
                break;
            }
            
            if(m < 0 || f < 0){
                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.Scanner;

public class Main {
	public static void main(String [] args){
        Scanner scan = new Scanner(System.in);
        
        while(true){
            int m = scan.nextInt();
            int f = scan.nextInt();
            int r = scan.nextInt();
            
            if(m < 0 && f < 0 && r < 0){
                break;
            }
            
            if(m < 0 || f < 0){
                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');
            }
        }
    }
}