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

public class Main{
	public static void main(String[] args) throws IOException{ 
		BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
		while(true){
			String line = br.readLine();
			String[] spl = line.split(" ");
			int m = Integer.parseInt(spl[0]);
			int f = Integer.parseInt(spl[1]);
			int r = Integer.parseInt(spl[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 || ( m + f >=30) && r >= 50){
				System.out.println("C");
			}else if( m + f >= 30){
				System.out.println("D");
			}else{
				System.out.println("F");
			}
		}
	}
} import java.io.*; io.* java import java.util.Arrays; Arrays java.util public class Main{
	public static void main(String[] args) throws IOException{ 
		BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
		while(true){
			String line = br.readLine();
			String[] spl = line.split(" ");
			int m = Integer.parseInt(spl[0]);
			int f = Integer.parseInt(spl[1]);
			int r = Integer.parseInt(spl[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 || ( m + f >=30) && r >= 50){
				System.out.println("C");
			}else if( m + f >= 30){
				System.out.println("D");
			}else{
				System.out.println("F");
			}
		}
	}
} Main public static void main(String[] args) throws IOException{ 
		BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
		while(true){
			String line = br.readLine();
			String[] spl = line.split(" ");
			int m = Integer.parseInt(spl[0]);
			int f = Integer.parseInt(spl[1]);
			int r = Integer.parseInt(spl[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 || ( m + f >=30) && r >= 50){
				System.out.println("C");
			}else if( m + f >= 30){
				System.out.println("D");
			}else{
				System.out.println("F");
			}
		}
	} main { 
		BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
		while(true){
			String line = br.readLine();
			String[] spl = line.split(" ");
			int m = Integer.parseInt(spl[0]);
			int f = Integer.parseInt(spl[1]);
			int r = Integer.parseInt(spl[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 || ( m + f >=30) && r >= 50){
				System.out.println("C");
			}else if( m + f >= 30){
				System.out.println("D");
			}else{
				System.out.println("F");
			}
		}
	} BufferedReader br = new BufferedReader(new InputStreamReader(System.in)); br new BufferedReader(new InputStreamReader(System.in)) while(true){
			String line = br.readLine();
			String[] spl = line.split(" ");
			int m = Integer.parseInt(spl[0]);
			int f = Integer.parseInt(spl[1]);
			int r = Integer.parseInt(spl[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 || ( m + f >=30) && r >= 50){
				System.out.println("C");
			}else if( m + f >= 30){
				System.out.println("D");
			}else{
				System.out.println("F");
			}
		} true {
			String line = br.readLine();
			String[] spl = line.split(" ");
			int m = Integer.parseInt(spl[0]);
			int f = Integer.parseInt(spl[1]);
			int r = Integer.parseInt(spl[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 || ( m + f >=30) && r >= 50){
				System.out.println("C");
			}else if( m + f >= 30){
				System.out.println("D");
			}else{
				System.out.println("F");
			}
		} String line = br.readLine(); line br.readLine() br.readLine br String[] spl = line.split(" "); spl line.split(" ") line.split line " " int m = Integer.parseInt(spl[0]); m Integer.parseInt(spl[0]) Integer.parseInt Integer spl[0] spl 0 int f = Integer.parseInt(spl[1]); f Integer.parseInt(spl[1]) Integer.parseInt Integer spl[1] spl 1 int r = Integer.parseInt(spl[2]); r Integer.parseInt(spl[2]) Integer.parseInt Integer spl[2] spl 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 || ( m + f >=30) && r >= 50){
				System.out.println("C");
			}else if( m + f >= 30){
				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 || ( m + f >=30) && r >= 50){
				System.out.println("C");
			}else if( m + f >= 30){
				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 || ( m + f >=30) && r >= 50){
				System.out.println("C");
			}else if( m + f >= 30){
				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 || ( m + f >=30) && r >= 50){
				System.out.println("C");
			}else if( m + f >= 30){
				System.out.println("D");
			}else{
				System.out.println("F");
			} m + f >= 50 || ( m + f >=30) && r >= 50 m + f >= 50 m + f m f 50 ( m + f >=30) && r >= 50 ( m + f >=30) m + f m f 30 r >= 50 r 50 {
				System.out.println("C");
			} System.out.println("C") System.out.println System.out System System.out "C" if( m + f >= 30){
				System.out.println("D");
			}else{
				System.out.println("F");
			} m + f >= 30 m + f m f 30 {
				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{ 
		BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
		while(true){
			String line = br.readLine();
			String[] spl = line.split(" ");
			int m = Integer.parseInt(spl[0]);
			int f = Integer.parseInt(spl[1]);
			int r = Integer.parseInt(spl[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 || ( m + f >=30) && r >= 50){
				System.out.println("C");
			}else if( m + f >= 30){
				System.out.println("D");
			}else{
				System.out.println("F");
			}
		}
	}
} public class Main{
	public static void main(String[] args) throws IOException{ 
		BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
		while(true){
			String line = br.readLine();
			String[] spl = line.split(" ");
			int m = Integer.parseInt(spl[0]);
			int f = Integer.parseInt(spl[1]);
			int r = Integer.parseInt(spl[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 || ( m + f >=30) && r >= 50){
				System.out.println("C");
			}else if( m + f >= 30){
				System.out.println("D");
			}else{
				System.out.println("F");
			}
		}
	}
} Main
import java.io.*;
import java.util.Arrays;

public class Main{
	public static void main(String[] args) throws IOException{ 
		BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
		while(true){
			String line = br.readLine();
			String[] spl = line.split(" ");
			int m = Integer.parseInt(spl[0]);
			int f = Integer.parseInt(spl[1]);
			int r = Integer.parseInt(spl[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 || ( m + f >=30) && r >= 50){
				System.out.println("C");
			}else if( m + f >= 30){
				System.out.println("D");
			}else{
				System.out.println("F");
			}
		}
	}
}