import java.util.Scanner;

public class Main{
    public static void main(String[]args){
        Scanner scn = new Scanner(System.in);
        
        while(true){
            int m=scn.nextInt();
            int f=scn.nextInt();
            int r=scn.nextInt();
            
            if(m==-1 && f==-1 && r==-1){
                break;
            } else {
                transcript(m,f,r);
            }
        }
    }

    private static void transcript(int m, int f, int r) {
        int sum=m+f;
        if(m==-1 || f==-1){
            System.out.println("F");
        }else if(sum >= 80){
            System.out.println("A");
        }else if(sum>=65 && sum<80){
            System.out.println("B");
        }else if(sum>=50 && sum<65){
            System.out.println("C");
        }else if(sum>=30 && sum<50){
            if(r>=50){
                System.out.println("C");
            }else{
                System.out.println("D");
            }
        }else if(sum>=0 && sum<30){
            System.out.println("F");
        }
    }
} import java.util.Scanner; Scanner java.util public class Main{
    public static void main(String[]args){
        Scanner scn = new Scanner(System.in);
        
        while(true){
            int m=scn.nextInt();
            int f=scn.nextInt();
            int r=scn.nextInt();
            
            if(m==-1 && f==-1 && r==-1){
                break;
            } else {
                transcript(m,f,r);
            }
        }
    }

    private static void transcript(int m, int f, int r) {
        int sum=m+f;
        if(m==-1 || f==-1){
            System.out.println("F");
        }else if(sum >= 80){
            System.out.println("A");
        }else if(sum>=65 && sum<80){
            System.out.println("B");
        }else if(sum>=50 && sum<65){
            System.out.println("C");
        }else if(sum>=30 && sum<50){
            if(r>=50){
                System.out.println("C");
            }else{
                System.out.println("D");
            }
        }else if(sum>=0 && sum<30){
            System.out.println("F");
        }
    }
} Main public static void main(String[]args){
        Scanner scn = new Scanner(System.in);
        
        while(true){
            int m=scn.nextInt();
            int f=scn.nextInt();
            int r=scn.nextInt();
            
            if(m==-1 && f==-1 && r==-1){
                break;
            } else {
                transcript(m,f,r);
            }
        }
    } main {
        Scanner scn = new Scanner(System.in);
        
        while(true){
            int m=scn.nextInt();
            int f=scn.nextInt();
            int r=scn.nextInt();
            
            if(m==-1 && f==-1 && r==-1){
                break;
            } else {
                transcript(m,f,r);
            }
        }
    } Scanner scn = new Scanner(System.in); scn new Scanner(System.in) while(true){
            int m=scn.nextInt();
            int f=scn.nextInt();
            int r=scn.nextInt();
            
            if(m==-1 && f==-1 && r==-1){
                break;
            } else {
                transcript(m,f,r);
            }
        } true {
            int m=scn.nextInt();
            int f=scn.nextInt();
            int r=scn.nextInt();
            
            if(m==-1 && f==-1 && r==-1){
                break;
            } else {
                transcript(m,f,r);
            }
        } int m=scn.nextInt(); m scn.nextInt() scn.nextInt scn int f=scn.nextInt(); f scn.nextInt() scn.nextInt scn int r=scn.nextInt(); r scn.nextInt() scn.nextInt scn if(m==-1 && f==-1 && r==-1){
                break;
            } else {
                transcript(m,f,r);
            } 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; {
                transcript(m,f,r);
            } transcript(m,f,r) transcript m f r String[]args args private static void transcript(int m, int f, int r) {
        int sum=m+f;
        if(m==-1 || f==-1){
            System.out.println("F");
        }else if(sum >= 80){
            System.out.println("A");
        }else if(sum>=65 && sum<80){
            System.out.println("B");
        }else if(sum>=50 && sum<65){
            System.out.println("C");
        }else if(sum>=30 && sum<50){
            if(r>=50){
                System.out.println("C");
            }else{
                System.out.println("D");
            }
        }else if(sum>=0 && sum<30){
            System.out.println("F");
        }
    } transcript {
        int sum=m+f;
        if(m==-1 || f==-1){
            System.out.println("F");
        }else if(sum >= 80){
            System.out.println("A");
        }else if(sum>=65 && sum<80){
            System.out.println("B");
        }else if(sum>=50 && sum<65){
            System.out.println("C");
        }else if(sum>=30 && sum<50){
            if(r>=50){
                System.out.println("C");
            }else{
                System.out.println("D");
            }
        }else if(sum>=0 && sum<30){
            System.out.println("F");
        }
    } int sum=m+f; sum m+f m f if(m==-1 || f==-1){
            System.out.println("F");
        }else if(sum >= 80){
            System.out.println("A");
        }else if(sum>=65 && sum<80){
            System.out.println("B");
        }else if(sum>=50 && sum<65){
            System.out.println("C");
        }else if(sum>=30 && sum<50){
            if(r>=50){
                System.out.println("C");
            }else{
                System.out.println("D");
            }
        }else if(sum>=0 && sum<30){
            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(sum >= 80){
            System.out.println("A");
        }else if(sum>=65 && sum<80){
            System.out.println("B");
        }else if(sum>=50 && sum<65){
            System.out.println("C");
        }else if(sum>=30 && sum<50){
            if(r>=50){
                System.out.println("C");
            }else{
                System.out.println("D");
            }
        }else if(sum>=0 && sum<30){
            System.out.println("F");
        } sum >= 80 sum 80 {
            System.out.println("A");
        } System.out.println("A") System.out.println System.out System System.out "A" if(sum>=65 && sum<80){
            System.out.println("B");
        }else if(sum>=50 && sum<65){
            System.out.println("C");
        }else if(sum>=30 && sum<50){
            if(r>=50){
                System.out.println("C");
            }else{
                System.out.println("D");
            }
        }else if(sum>=0 && sum<30){
            System.out.println("F");
        } sum>=65 && sum<80 sum>=65 sum 65 sum<80 sum 80 {
            System.out.println("B");
        } System.out.println("B") System.out.println System.out System System.out "B" if(sum>=50 && sum<65){
            System.out.println("C");
        }else if(sum>=30 && sum<50){
            if(r>=50){
                System.out.println("C");
            }else{
                System.out.println("D");
            }
        }else if(sum>=0 && sum<30){
            System.out.println("F");
        } sum>=50 && sum<65 sum>=50 sum 50 sum<65 sum 65 {
            System.out.println("C");
        } System.out.println("C") System.out.println System.out System System.out "C" if(sum>=30 && sum<50){
            if(r>=50){
                System.out.println("C");
            }else{
                System.out.println("D");
            }
        }else if(sum>=0 && sum<30){
            System.out.println("F");
        } sum>=30 && sum<50 sum>=30 sum 30 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(sum>=0 && sum<30){
            System.out.println("F");
        } sum>=0 && sum<30 sum>=0 sum 0 sum<30 sum 30 {
            System.out.println("F");
        } System.out.println("F") System.out.println System.out System System.out "F" int m m int f f int r r public class Main{
    public static void main(String[]args){
        Scanner scn = new Scanner(System.in);
        
        while(true){
            int m=scn.nextInt();
            int f=scn.nextInt();
            int r=scn.nextInt();
            
            if(m==-1 && f==-1 && r==-1){
                break;
            } else {
                transcript(m,f,r);
            }
        }
    }

    private static void transcript(int m, int f, int r) {
        int sum=m+f;
        if(m==-1 || f==-1){
            System.out.println("F");
        }else if(sum >= 80){
            System.out.println("A");
        }else if(sum>=65 && sum<80){
            System.out.println("B");
        }else if(sum>=50 && sum<65){
            System.out.println("C");
        }else if(sum>=30 && sum<50){
            if(r>=50){
                System.out.println("C");
            }else{
                System.out.println("D");
            }
        }else if(sum>=0 && sum<30){
            System.out.println("F");
        }
    }
} public class Main{
    public static void main(String[]args){
        Scanner scn = new Scanner(System.in);
        
        while(true){
            int m=scn.nextInt();
            int f=scn.nextInt();
            int r=scn.nextInt();
            
            if(m==-1 && f==-1 && r==-1){
                break;
            } else {
                transcript(m,f,r);
            }
        }
    }

    private static void transcript(int m, int f, int r) {
        int sum=m+f;
        if(m==-1 || f==-1){
            System.out.println("F");
        }else if(sum >= 80){
            System.out.println("A");
        }else if(sum>=65 && sum<80){
            System.out.println("B");
        }else if(sum>=50 && sum<65){
            System.out.println("C");
        }else if(sum>=30 && sum<50){
            if(r>=50){
                System.out.println("C");
            }else{
                System.out.println("D");
            }
        }else if(sum>=0 && sum<30){
            System.out.println("F");
        }
    }
} Main
import java.util.Scanner;

public class Main{
    public static void main(String[]args){
        Scanner scn = new Scanner(System.in);
        
        while(true){
            int m=scn.nextInt();
            int f=scn.nextInt();
            int r=scn.nextInt();
            
            if(m==-1 && f==-1 && r==-1){
                break;
            } else {
                transcript(m,f,r);
            }
        }
    }

    private static void transcript(int m, int f, int r) {
        int sum=m+f;
        if(m==-1 || f==-1){
            System.out.println("F");
        }else if(sum >= 80){
            System.out.println("A");
        }else if(sum>=65 && sum<80){
            System.out.println("B");
        }else if(sum>=50 && sum<65){
            System.out.println("C");
        }else if(sum>=30 && sum<50){
            if(r>=50){
                System.out.println("C");
            }else{
                System.out.println("D");
            }
        }else if(sum>=0 && sum<30){
            System.out.println("F");
        }
    }
}