import java.io.*;
import java.util.*;

class Main{
    public static void main(String[] args)throws IOException{
        int m, f, r;

        InputStreamReader isr =new InputStreamReader(System.in);
   		BufferedReader br =new BufferedReader(isr);
    	String buf = br.readLine();
        StringTokenizer st = new StringTokenizer(buf," ");
        m = Integer.parseInt(st.nextToken());
        f = Integer.parseInt(st.nextToken());
        r = Integer.parseInt(st.nextToken());
        
        while(true) {
            if (m == -1 && f == -1 && r==-1) break;
            else if (m == -1 || f == -1) {
                System.out.println("F");
            }
            else if (m + f >= 80) {
                System.out.println("A");
            }
            else if (m + f < 80 && m + f >= 65) {
                System.out.println("B");
            }
            else if (m + f < 65 && m + f >= 50) {
                System.out.println("C");
            }
            else if (m + f < 50 && m + f >= 30) {
                if (r >= 50) System.out.println("C");
                else System.out.println("D");
            }
            else System.out.println("F");
            
            buf = br.readLine();
            st = new StringTokenizer(buf," ");
            m = Integer.parseInt(st.nextToken());
            f = Integer.parseInt(st.nextToken());
            r = Integer.parseInt(st.nextToken());
        }
    }
} import java.io.*; io.* java import java.util.*; util.* java class Main{
    public static void main(String[] args)throws IOException{
        int m, f, r;

        InputStreamReader isr =new InputStreamReader(System.in);
   		BufferedReader br =new BufferedReader(isr);
    	String buf = br.readLine();
        StringTokenizer st = new StringTokenizer(buf," ");
        m = Integer.parseInt(st.nextToken());
        f = Integer.parseInt(st.nextToken());
        r = Integer.parseInt(st.nextToken());
        
        while(true) {
            if (m == -1 && f == -1 && r==-1) break;
            else if (m == -1 || f == -1) {
                System.out.println("F");
            }
            else if (m + f >= 80) {
                System.out.println("A");
            }
            else if (m + f < 80 && m + f >= 65) {
                System.out.println("B");
            }
            else if (m + f < 65 && m + f >= 50) {
                System.out.println("C");
            }
            else if (m + f < 50 && m + f >= 30) {
                if (r >= 50) System.out.println("C");
                else System.out.println("D");
            }
            else System.out.println("F");
            
            buf = br.readLine();
            st = new StringTokenizer(buf," ");
            m = Integer.parseInt(st.nextToken());
            f = Integer.parseInt(st.nextToken());
            r = Integer.parseInt(st.nextToken());
        }
    }
} Main public static void main(String[] args)throws IOException{
        int m, f, r;

        InputStreamReader isr =new InputStreamReader(System.in);
   		BufferedReader br =new BufferedReader(isr);
    	String buf = br.readLine();
        StringTokenizer st = new StringTokenizer(buf," ");
        m = Integer.parseInt(st.nextToken());
        f = Integer.parseInt(st.nextToken());
        r = Integer.parseInt(st.nextToken());
        
        while(true) {
            if (m == -1 && f == -1 && r==-1) break;
            else if (m == -1 || f == -1) {
                System.out.println("F");
            }
            else if (m + f >= 80) {
                System.out.println("A");
            }
            else if (m + f < 80 && m + f >= 65) {
                System.out.println("B");
            }
            else if (m + f < 65 && m + f >= 50) {
                System.out.println("C");
            }
            else if (m + f < 50 && m + f >= 30) {
                if (r >= 50) System.out.println("C");
                else System.out.println("D");
            }
            else System.out.println("F");
            
            buf = br.readLine();
            st = new StringTokenizer(buf," ");
            m = Integer.parseInt(st.nextToken());
            f = Integer.parseInt(st.nextToken());
            r = Integer.parseInt(st.nextToken());
        }
    } main {
        int m, f, r;

        InputStreamReader isr =new InputStreamReader(System.in);
   		BufferedReader br =new BufferedReader(isr);
    	String buf = br.readLine();
        StringTokenizer st = new StringTokenizer(buf," ");
        m = Integer.parseInt(st.nextToken());
        f = Integer.parseInt(st.nextToken());
        r = Integer.parseInt(st.nextToken());
        
        while(true) {
            if (m == -1 && f == -1 && r==-1) break;
            else if (m == -1 || f == -1) {
                System.out.println("F");
            }
            else if (m + f >= 80) {
                System.out.println("A");
            }
            else if (m + f < 80 && m + f >= 65) {
                System.out.println("B");
            }
            else if (m + f < 65 && m + f >= 50) {
                System.out.println("C");
            }
            else if (m + f < 50 && m + f >= 30) {
                if (r >= 50) System.out.println("C");
                else System.out.println("D");
            }
            else System.out.println("F");
            
            buf = br.readLine();
            st = new StringTokenizer(buf," ");
            m = Integer.parseInt(st.nextToken());
            f = Integer.parseInt(st.nextToken());
            r = Integer.parseInt(st.nextToken());
        }
    } int m m f f r; r InputStreamReader isr =new InputStreamReader(System.in); isr new InputStreamReader(System.in) BufferedReader br =new BufferedReader(isr); br new BufferedReader(isr) String buf = br.readLine(); buf br.readLine() br.readLine br StringTokenizer st = new StringTokenizer(buf," "); st new StringTokenizer(buf," ") m = Integer.parseInt(st.nextToken()) m Integer.parseInt(st.nextToken()) Integer.parseInt Integer st.nextToken() st.nextToken st f = Integer.parseInt(st.nextToken()) f Integer.parseInt(st.nextToken()) Integer.parseInt Integer st.nextToken() st.nextToken st r = Integer.parseInt(st.nextToken()) r Integer.parseInt(st.nextToken()) Integer.parseInt Integer st.nextToken() st.nextToken st while(true) {
            if (m == -1 && f == -1 && r==-1) break;
            else if (m == -1 || f == -1) {
                System.out.println("F");
            }
            else if (m + f >= 80) {
                System.out.println("A");
            }
            else if (m + f < 80 && m + f >= 65) {
                System.out.println("B");
            }
            else if (m + f < 65 && m + f >= 50) {
                System.out.println("C");
            }
            else if (m + f < 50 && m + f >= 30) {
                if (r >= 50) System.out.println("C");
                else System.out.println("D");
            }
            else System.out.println("F");
            
            buf = br.readLine();
            st = new StringTokenizer(buf," ");
            m = Integer.parseInt(st.nextToken());
            f = Integer.parseInt(st.nextToken());
            r = Integer.parseInt(st.nextToken());
        } true {
            if (m == -1 && f == -1 && r==-1) break;
            else if (m == -1 || f == -1) {
                System.out.println("F");
            }
            else if (m + f >= 80) {
                System.out.println("A");
            }
            else if (m + f < 80 && m + f >= 65) {
                System.out.println("B");
            }
            else if (m + f < 65 && m + f >= 50) {
                System.out.println("C");
            }
            else if (m + f < 50 && m + f >= 30) {
                if (r >= 50) System.out.println("C");
                else System.out.println("D");
            }
            else System.out.println("F");
            
            buf = br.readLine();
            st = new StringTokenizer(buf," ");
            m = Integer.parseInt(st.nextToken());
            f = Integer.parseInt(st.nextToken());
            r = Integer.parseInt(st.nextToken());
        } if (m == -1 && f == -1 && r==-1) break;
            else if (m == -1 || f == -1) {
                System.out.println("F");
            }
            else if (m + f >= 80) {
                System.out.println("A");
            }
            else if (m + f < 80 && m + f >= 65) {
                System.out.println("B");
            }
            else if (m + f < 65 && m + f >= 50) {
                System.out.println("C");
            }
            else if (m + f < 50 && m + f >= 30) {
                if (r >= 50) System.out.println("C");
                else System.out.println("D");
            }
            else System.out.println("F"); 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 < 80 && m + f >= 65) {
                System.out.println("B");
            }
            else if (m + f < 65 && m + f >= 50) {
                System.out.println("C");
            }
            else if (m + f < 50 && 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 < 80 && m + f >= 65) {
                System.out.println("B");
            }
            else if (m + f < 65 && m + f >= 50) {
                System.out.println("C");
            }
            else if (m + f < 50 && 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 < 80 && m + f >= 65) {
                System.out.println("B");
            }
            else if (m + f < 65 && m + f >= 50) {
                System.out.println("C");
            }
            else if (m + f < 50 && 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 >= 65 m + f < 80 m + f m f 80 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 < 65 && m + f >= 50) {
                System.out.println("C");
            }
            else if (m + f < 50 && 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 >= 50 m + f < 65 m + f m f 65 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 < 50 && 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 >= 30 m + f < 50 m + f m f 50 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 System.out System System.out "C" System.out.println("D") System.out.println System.out System System.out "D" System.out.println("F") System.out.println System.out System System.out "F" buf = br.readLine() buf br.readLine() br.readLine br st = new StringTokenizer(buf," ") st new StringTokenizer(buf," ") m = Integer.parseInt(st.nextToken()) m Integer.parseInt(st.nextToken()) Integer.parseInt Integer st.nextToken() st.nextToken st f = Integer.parseInt(st.nextToken()) f Integer.parseInt(st.nextToken()) Integer.parseInt Integer st.nextToken() st.nextToken st r = Integer.parseInt(st.nextToken()) r Integer.parseInt(st.nextToken()) Integer.parseInt Integer st.nextToken() st.nextToken st String[] args args
import java.io.*;
import java.util.*;

class Main{
    public static void main(String[] args)throws IOException{
        int m, f, r;

        InputStreamReader isr =new InputStreamReader(System.in);
   		BufferedReader br =new BufferedReader(isr);
    	String buf = br.readLine();
        StringTokenizer st = new StringTokenizer(buf," ");
        m = Integer.parseInt(st.nextToken());
        f = Integer.parseInt(st.nextToken());
        r = Integer.parseInt(st.nextToken());
        
        while(true) {
            if (m == -1 && f == -1 && r==-1) break;
            else if (m == -1 || f == -1) {
                System.out.println("F");
            }
            else if (m + f >= 80) {
                System.out.println("A");
            }
            else if (m + f < 80 && m + f >= 65) {
                System.out.println("B");
            }
            else if (m + f < 65 && m + f >= 50) {
                System.out.println("C");
            }
            else if (m + f < 50 && m + f >= 30) {
                if (r >= 50) System.out.println("C");
                else System.out.println("D");
            }
            else System.out.println("F");
            
            buf = br.readLine();
            st = new StringTokenizer(buf," ");
            m = Integer.parseInt(st.nextToken());
            f = Integer.parseInt(st.nextToken());
            r = Integer.parseInt(st.nextToken());
        }
    }
}