import java.io.*;
public class Main {
	public static void main(String[] args)throws IOException {
		int m = 0,f=0,r=0;
		BufferedReader br=new BufferedReader(new InputStreamReader(System.in));
		while(true){
			String str = br.readLine();
			String[] i=str.split(" ");
				m=Integer.parseInt(i[0]);
				f=Integer.parseInt(i[1]);
				r=Integer.parseInt(i[2]);
				if(m==(-1)&&f==(-1)&&r==(-1)){break;}
				if(m==(-1)||f==(-1)){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.io.*; io.* java public class Main {
	public static void main(String[] args)throws IOException {
		int m = 0,f=0,r=0;
		BufferedReader br=new BufferedReader(new InputStreamReader(System.in));
		while(true){
			String str = br.readLine();
			String[] i=str.split(" ");
				m=Integer.parseInt(i[0]);
				f=Integer.parseInt(i[1]);
				r=Integer.parseInt(i[2]);
				if(m==(-1)&&f==(-1)&&r==(-1)){break;}
				if(m==(-1)||f==(-1)){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)throws IOException {
		int m = 0,f=0,r=0;
		BufferedReader br=new BufferedReader(new InputStreamReader(System.in));
		while(true){
			String str = br.readLine();
			String[] i=str.split(" ");
				m=Integer.parseInt(i[0]);
				f=Integer.parseInt(i[1]);
				r=Integer.parseInt(i[2]);
				if(m==(-1)&&f==(-1)&&r==(-1)){break;}
				if(m==(-1)||f==(-1)){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 {
		int m = 0,f=0,r=0;
		BufferedReader br=new BufferedReader(new InputStreamReader(System.in));
		while(true){
			String str = br.readLine();
			String[] i=str.split(" ");
				m=Integer.parseInt(i[0]);
				f=Integer.parseInt(i[1]);
				r=Integer.parseInt(i[2]);
				if(m==(-1)&&f==(-1)&&r==(-1)){break;}
				if(m==(-1)||f==(-1)){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 = 0 m 0 f=0 f 0 r=0; r 0 BufferedReader br=new BufferedReader(new InputStreamReader(System.in)); br new BufferedReader(new InputStreamReader(System.in)) while(true){
			String str = br.readLine();
			String[] i=str.split(" ");
				m=Integer.parseInt(i[0]);
				f=Integer.parseInt(i[1]);
				r=Integer.parseInt(i[2]);
				if(m==(-1)&&f==(-1)&&r==(-1)){break;}
				if(m==(-1)||f==(-1)){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 {
			String str = br.readLine();
			String[] i=str.split(" ");
				m=Integer.parseInt(i[0]);
				f=Integer.parseInt(i[1]);
				r=Integer.parseInt(i[2]);
				if(m==(-1)&&f==(-1)&&r==(-1)){break;}
				if(m==(-1)||f==(-1)){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");}
		} String str = br.readLine(); str br.readLine() br.readLine br String[] i=str.split(" "); i str.split(" ") str.split str " " m=Integer.parseInt(i[0]) m Integer.parseInt(i[0]) Integer.parseInt Integer i[0] i 0 f=Integer.parseInt(i[1]) f Integer.parseInt(i[1]) Integer.parseInt Integer i[1] i 1 r=Integer.parseInt(i[2]) r Integer.parseInt(i[2]) Integer.parseInt Integer i[2] i 2 if(m==(-1)&&f==(-1)&&r==(-1)){break;} 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; if(m==(-1)||f==(-1)){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==(-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>=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)throws IOException {
		int m = 0,f=0,r=0;
		BufferedReader br=new BufferedReader(new InputStreamReader(System.in));
		while(true){
			String str = br.readLine();
			String[] i=str.split(" ");
				m=Integer.parseInt(i[0]);
				f=Integer.parseInt(i[1]);
				r=Integer.parseInt(i[2]);
				if(m==(-1)&&f==(-1)&&r==(-1)){break;}
				if(m==(-1)||f==(-1)){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)throws IOException {
		int m = 0,f=0,r=0;
		BufferedReader br=new BufferedReader(new InputStreamReader(System.in));
		while(true){
			String str = br.readLine();
			String[] i=str.split(" ");
				m=Integer.parseInt(i[0]);
				f=Integer.parseInt(i[1]);
				r=Integer.parseInt(i[2]);
				if(m==(-1)&&f==(-1)&&r==(-1)){break;}
				if(m==(-1)||f==(-1)){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.io.*;
public class Main {
	public static void main(String[] args)throws IOException {
		int m = 0,f=0,r=0;
		BufferedReader br=new BufferedReader(new InputStreamReader(System.in));
		while(true){
			String str = br.readLine();
			String[] i=str.split(" ");
				m=Integer.parseInt(i[0]);
				f=Integer.parseInt(i[1]);
				r=Integer.parseInt(i[2]);
				if(m==(-1)&&f==(-1)&&r==(-1)){break;}
				if(m==(-1)||f==(-1)){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");}
		}
	}
}