import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;

public class Main {
	public void maain(){

	}

	public static void main(String[] args) throws IOException {
		// TODO ?????????????????????????????????????????????

		//1???????????\????????¢
		InputStreamReader isr = new InputStreamReader(System.in);
        BufferedReader br = new BufferedReader(isr);


        while(true){
        	String[] input = br.readLine().split(" ");
            int m = Integer.parseInt(input[0]);
            int f = Integer.parseInt(input[1]);
            int r = Integer.parseInt(input[2]);

            //?????????????????????
            if(m == -1 && f == -1 && r == -1){
            	return;
            }

            //???????????¨???
            if(m + f < 30 || (m == -1 || f == -1)){
            	System.out.println("F");
            }else if(m + f >= 80){
            	System.out.println("A");
            }else if(m + f >= 65 && m + f <= 80){
            	System.out.println("B");
            }else if(m + f >= 50 && m + f <= 65){
            	System.out.println("C");
            }else if(m + f >= 30 && m + f <= 50){
            	if(r >= 50){
            		System.out.println("C");
            	}else{
                	System.out.println("D");
            	}
            }
        }
	}
} import java.io.BufferedReader; BufferedReader java.io import java.io.IOException; IOException java.io import java.io.InputStreamReader; InputStreamReader java.io public class Main {
	public void maain(){

	}

	public static void main(String[] args) throws IOException {
		// TODO ?????????????????????????????????????????????

		//1???????????\????????¢
		InputStreamReader isr = new InputStreamReader(System.in);
        BufferedReader br = new BufferedReader(isr);


        while(true){
        	String[] input = br.readLine().split(" ");
            int m = Integer.parseInt(input[0]);
            int f = Integer.parseInt(input[1]);
            int r = Integer.parseInt(input[2]);

            //?????????????????????
            if(m == -1 && f == -1 && r == -1){
            	return;
            }

            //???????????¨???
            if(m + f < 30 || (m == -1 || f == -1)){
            	System.out.println("F");
            }else if(m + f >= 80){
            	System.out.println("A");
            }else if(m + f >= 65 && m + f <= 80){
            	System.out.println("B");
            }else if(m + f >= 50 && m + f <= 65){
            	System.out.println("C");
            }else if(m + f >= 30 && m + f <= 50){
            	if(r >= 50){
            		System.out.println("C");
            	}else{
                	System.out.println("D");
            	}
            }
        }
	}
} Main public void maain(){

	} maain {

	} public static void main(String[] args) throws IOException {
		// TODO ?????????????????????????????????????????????

		//1???????????\????????¢
		InputStreamReader isr = new InputStreamReader(System.in);
        BufferedReader br = new BufferedReader(isr);


        while(true){
        	String[] input = br.readLine().split(" ");
            int m = Integer.parseInt(input[0]);
            int f = Integer.parseInt(input[1]);
            int r = Integer.parseInt(input[2]);

            //?????????????????????
            if(m == -1 && f == -1 && r == -1){
            	return;
            }

            //???????????¨???
            if(m + f < 30 || (m == -1 || f == -1)){
            	System.out.println("F");
            }else if(m + f >= 80){
            	System.out.println("A");
            }else if(m + f >= 65 && m + f <= 80){
            	System.out.println("B");
            }else if(m + f >= 50 && m + f <= 65){
            	System.out.println("C");
            }else if(m + f >= 30 && m + f <= 50){
            	if(r >= 50){
            		System.out.println("C");
            	}else{
                	System.out.println("D");
            	}
            }
        }
	} main {
		// TODO ?????????????????????????????????????????????

		//1???????????\????????¢
		InputStreamReader isr = new InputStreamReader(System.in);
        BufferedReader br = new BufferedReader(isr);


        while(true){
        	String[] input = br.readLine().split(" ");
            int m = Integer.parseInt(input[0]);
            int f = Integer.parseInt(input[1]);
            int r = Integer.parseInt(input[2]);

            //?????????????????????
            if(m == -1 && f == -1 && r == -1){
            	return;
            }

            //???????????¨???
            if(m + f < 30 || (m == -1 || f == -1)){
            	System.out.println("F");
            }else if(m + f >= 80){
            	System.out.println("A");
            }else if(m + f >= 65 && m + f <= 80){
            	System.out.println("B");
            }else if(m + f >= 50 && m + f <= 65){
            	System.out.println("C");
            }else if(m + f >= 30 && m + f <= 50){
            	if(r >= 50){
            		System.out.println("C");
            	}else{
                	System.out.println("D");
            	}
            }
        }
	} InputStreamReader isr = new InputStreamReader(System.in); isr new InputStreamReader(System.in) BufferedReader br = new BufferedReader(isr); br new BufferedReader(isr) while(true){
        	String[] input = br.readLine().split(" ");
            int m = Integer.parseInt(input[0]);
            int f = Integer.parseInt(input[1]);
            int r = Integer.parseInt(input[2]);

            //?????????????????????
            if(m == -1 && f == -1 && r == -1){
            	return;
            }

            //???????????¨???
            if(m + f < 30 || (m == -1 || f == -1)){
            	System.out.println("F");
            }else if(m + f >= 80){
            	System.out.println("A");
            }else if(m + f >= 65 && m + f <= 80){
            	System.out.println("B");
            }else if(m + f >= 50 && m + f <= 65){
            	System.out.println("C");
            }else if(m + f >= 30 && m + f <= 50){
            	if(r >= 50){
            		System.out.println("C");
            	}else{
                	System.out.println("D");
            	}
            }
        } true {
        	String[] input = br.readLine().split(" ");
            int m = Integer.parseInt(input[0]);
            int f = Integer.parseInt(input[1]);
            int r = Integer.parseInt(input[2]);

            //?????????????????????
            if(m == -1 && f == -1 && r == -1){
            	return;
            }

            //???????????¨???
            if(m + f < 30 || (m == -1 || f == -1)){
            	System.out.println("F");
            }else if(m + f >= 80){
            	System.out.println("A");
            }else if(m + f >= 65 && m + f <= 80){
            	System.out.println("B");
            }else if(m + f >= 50 && m + f <= 65){
            	System.out.println("C");
            }else if(m + f >= 30 && m + f <= 50){
            	if(r >= 50){
            		System.out.println("C");
            	}else{
                	System.out.println("D");
            	}
            }
        } String[] input = br.readLine().split(" "); input br.readLine().split(" ") br.readLine().split br.readLine() br.readLine br " " int m = Integer.parseInt(input[0]); m Integer.parseInt(input[0]) Integer.parseInt Integer input[0] input 0 int f = Integer.parseInt(input[1]); f Integer.parseInt(input[1]) Integer.parseInt Integer input[1] input 1 int r = Integer.parseInt(input[2]); r Integer.parseInt(input[2]) Integer.parseInt Integer input[2] input 2 if(m == -1 && f == -1 && r == -1){
            	return;
            } 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 {
            	return;
            } return; if(m + f < 30 || (m == -1 || f == -1)){
            	System.out.println("F");
            }else if(m + f >= 80){
            	System.out.println("A");
            }else if(m + f >= 65 && m + f <= 80){
            	System.out.println("B");
            }else if(m + f >= 50 && m + f <= 65){
            	System.out.println("C");
            }else if(m + f >= 30 && m + f <= 50){
            	if(r >= 50){
            		System.out.println("C");
            	}else{
                	System.out.println("D");
            	}
            } m + f < 30 || (m == -1 || f == -1) m + f < 30 m + f m f 30 (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 && m + f <= 80){
            	System.out.println("B");
            }else if(m + f >= 50 && m + f <= 65){
            	System.out.println("C");
            }else if(m + f >= 30 && m + f <= 50){
            	if(r >= 50){
            		System.out.println("C");
            	}else{
                	System.out.println("D");
            	}
            } 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 && m + f <= 80){
            	System.out.println("B");
            }else if(m + f >= 50 && m + f <= 65){
            	System.out.println("C");
            }else if(m + f >= 30 && m + f <= 50){
            	if(r >= 50){
            		System.out.println("C");
            	}else{
                	System.out.println("D");
            	}
            } m + f >= 65 && m + f <= 80 m + f >= 65 m + f m f 65 m + f <= 80 m + f m f 80 {
            	System.out.println("B");
            } System.out.println("B") System.out.println System.out System System.out "B" if(m + f >= 50 && m + f <= 65){
            	System.out.println("C");
            }else if(m + f >= 30 && m + f <= 50){
            	if(r >= 50){
            		System.out.println("C");
            	}else{
                	System.out.println("D");
            	}
            } m + f >= 50 && m + f <= 65 m + f >= 50 m + f m f 50 m + f <= 65 m + f m f 65 {
            	System.out.println("C");
            } System.out.println("C") System.out.println System.out System System.out "C" if(m + f >= 30 && m + f <= 50){
            	if(r >= 50){
            		System.out.println("C");
            	}else{
                	System.out.println("D");
            	}
            } m + f >= 30 && m + f <= 50 m + f >= 30 m + f m f 30 m + f <= 50 m + f m f 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("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" String[] args args public class Main {
	public void maain(){

	}

	public static void main(String[] args) throws IOException {
		// TODO ?????????????????????????????????????????????

		//1???????????\????????¢
		InputStreamReader isr = new InputStreamReader(System.in);
        BufferedReader br = new BufferedReader(isr);


        while(true){
        	String[] input = br.readLine().split(" ");
            int m = Integer.parseInt(input[0]);
            int f = Integer.parseInt(input[1]);
            int r = Integer.parseInt(input[2]);

            //?????????????????????
            if(m == -1 && f == -1 && r == -1){
            	return;
            }

            //???????????¨???
            if(m + f < 30 || (m == -1 || f == -1)){
            	System.out.println("F");
            }else if(m + f >= 80){
            	System.out.println("A");
            }else if(m + f >= 65 && m + f <= 80){
            	System.out.println("B");
            }else if(m + f >= 50 && m + f <= 65){
            	System.out.println("C");
            }else if(m + f >= 30 && m + f <= 50){
            	if(r >= 50){
            		System.out.println("C");
            	}else{
                	System.out.println("D");
            	}
            }
        }
	}
} public class Main {
	public void maain(){

	}

	public static void main(String[] args) throws IOException {
		// TODO ?????????????????????????????????????????????

		//1???????????\????????¢
		InputStreamReader isr = new InputStreamReader(System.in);
        BufferedReader br = new BufferedReader(isr);


        while(true){
        	String[] input = br.readLine().split(" ");
            int m = Integer.parseInt(input[0]);
            int f = Integer.parseInt(input[1]);
            int r = Integer.parseInt(input[2]);

            //?????????????????????
            if(m == -1 && f == -1 && r == -1){
            	return;
            }

            //???????????¨???
            if(m + f < 30 || (m == -1 || f == -1)){
            	System.out.println("F");
            }else if(m + f >= 80){
            	System.out.println("A");
            }else if(m + f >= 65 && m + f <= 80){
            	System.out.println("B");
            }else if(m + f >= 50 && m + f <= 65){
            	System.out.println("C");
            }else if(m + f >= 30 && m + f <= 50){
            	if(r >= 50){
            		System.out.println("C");
            	}else{
                	System.out.println("D");
            	}
            }
        }
	}
} Main
import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;

public class Main {
	public void maain(){

	}

	public static void main(String[] args) throws IOException {
		// TODO ?????????????????????????????????????????????

		//1???????????\????????¢
		InputStreamReader isr = new InputStreamReader(System.in);
        BufferedReader br = new BufferedReader(isr);


        while(true){
        	String[] input = br.readLine().split(" ");
            int m = Integer.parseInt(input[0]);
            int f = Integer.parseInt(input[1]);
            int r = Integer.parseInt(input[2]);

            //?????????????????????
            if(m == -1 && f == -1 && r == -1){
            	return;
            }

            //???????????¨???
            if(m + f < 30 || (m == -1 || f == -1)){
            	System.out.println("F");
            }else if(m + f >= 80){
            	System.out.println("A");
            }else if(m + f >= 65 && m + f <= 80){
            	System.out.println("B");
            }else if(m + f >= 50 && m + f <= 65){
            	System.out.println("C");
            }else if(m + f >= 30 && m + f <= 50){
            	if(r >= 50){
            		System.out.println("C");
            	}else{
                	System.out.println("D");
            	}
            }
        }
	}
}