import java.util.Scanner;

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

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