
import java.io.BufferedReader;
import java.io.InputStreamReader;
import java.util.StringTokenizer;

public class Main {
	public static void main(String[] args) throws Exception {
	
		try{
			
	BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
    String buf = br.readLine();
	
    int m, f, r;
    
    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());
    }
    
} catch (Exception e) {

	System.out.println("????????????");
	
}
		 }
    } import java.io.BufferedReader; BufferedReader java.io import java.io.InputStreamReader; InputStreamReader java.io import java.util.StringTokenizer; StringTokenizer java.util public class Main {
	public static void main(String[] args) throws Exception {
	
		try{
			
	BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
    String buf = br.readLine();
	
    int m, f, r;
    
    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());
    }
    
} catch (Exception e) {

	System.out.println("????????????");
	
}
		 }
    } Main public static void main(String[] args) throws Exception {
	
		try{
			
	BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
    String buf = br.readLine();
	
    int m, f, r;
    
    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());
    }
    
} catch (Exception e) {

	System.out.println("????????????");
	
}
		 } main {
	
		try{
			
	BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
    String buf = br.readLine();
	
    int m, f, r;
    
    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());
    }
    
} catch (Exception e) {

	System.out.println("????????????");
	
}
		 } try{
			
	BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
    String buf = br.readLine();
	
    int m, f, r;
    
    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());
    }
    
} catch (Exception e) {

	System.out.println("????????????");
	
} catch (Exception e) {

	System.out.println("????????????");
	
} Exception e {

	System.out.println("????????????");
	
} System.out.println("????????????") System.out.println System.out System System.out "????????????" {
			
	BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
    String buf = br.readLine();
	
    int m, f, r;
    
    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());
    }
    
} BufferedReader br = new BufferedReader(new InputStreamReader(System.in)); br new BufferedReader(new InputStreamReader(System.in)) String buf = br.readLine(); buf br.readLine() br.readLine br int m m f f r; r 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 public class Main {
	public static void main(String[] args) throws Exception {
	
		try{
			
	BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
    String buf = br.readLine();
	
    int m, f, r;
    
    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());
    }
    
} catch (Exception e) {

	System.out.println("????????????");
	
}
		 }
    } public class Main {
	public static void main(String[] args) throws Exception {
	
		try{
			
	BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
    String buf = br.readLine();
	
    int m, f, r;
    
    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());
    }
    
} catch (Exception e) {

	System.out.println("????????????");
	
}
		 }
    } Main

import java.io.BufferedReader;
import java.io.InputStreamReader;
import java.util.StringTokenizer;

public class Main {
	public static void main(String[] args) throws Exception {
	
		try{
			
	BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
    String buf = br.readLine();
	
    int m, f, r;
    
    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());
    }
    
} catch (Exception e) {

	System.out.println("????????????");
	
}
		 }
    }