import java.util.*;
public class Main {
	public static void main(String[] args) {
		Scanner scan = new Scanner(System.in);
		int m = 0;	//中間試験
		int f = 0;	//期末試験
		int r = 0;	//再試験
		int sum;//中間試験と期末試験の点数の合計
        while(true){
			 m = Integer.parseInt(scan.next());
			 f = Integer.parseInt(scan.next());
			 r = Integer.parseInt(scan.next());
			 sum = m + f;
			 if(m == -1 && f ==-1 && r == -1){
				 break;
			 }else{
				 if(m == -1 || f == -1){
					 System.out.println("F");
				 }else if(sum >= 80){
					 System.out.println("A");
				 }else if(sum >= 65 && sum <= 80){
					 System.out.println("B");
				 }else if(sum >= 50 && sum <= 65){
					 System.out.println("C");
				 }else if(sum >= 30 && sum <= 50){
					 if(r >= 50){
						 System.out.println("C");
					 }else{
						 System.out.println("D");
					 }
				 }else{
					 System.out.println("F");
				 }//if
			 }//if
		 }//while 
	}
} import java.util.*; util.* java public class Main {
	public static void main(String[] args) {
		Scanner scan = new Scanner(System.in);
		int m = 0;	//中間試験
		int f = 0;	//期末試験
		int r = 0;	//再試験
		int sum;//中間試験と期末試験の点数の合計
        while(true){
			 m = Integer.parseInt(scan.next());
			 f = Integer.parseInt(scan.next());
			 r = Integer.parseInt(scan.next());
			 sum = m + f;
			 if(m == -1 && f ==-1 && r == -1){
				 break;
			 }else{
				 if(m == -1 || f == -1){
					 System.out.println("F");
				 }else if(sum >= 80){
					 System.out.println("A");
				 }else if(sum >= 65 && sum <= 80){
					 System.out.println("B");
				 }else if(sum >= 50 && sum <= 65){
					 System.out.println("C");
				 }else if(sum >= 30 && sum <= 50){
					 if(r >= 50){
						 System.out.println("C");
					 }else{
						 System.out.println("D");
					 }
				 }else{
					 System.out.println("F");
				 }//if
			 }//if
		 }//while 
	}
} Main public static void main(String[] args) {
		Scanner scan = new Scanner(System.in);
		int m = 0;	//中間試験
		int f = 0;	//期末試験
		int r = 0;	//再試験
		int sum;//中間試験と期末試験の点数の合計
        while(true){
			 m = Integer.parseInt(scan.next());
			 f = Integer.parseInt(scan.next());
			 r = Integer.parseInt(scan.next());
			 sum = m + f;
			 if(m == -1 && f ==-1 && r == -1){
				 break;
			 }else{
				 if(m == -1 || f == -1){
					 System.out.println("F");
				 }else if(sum >= 80){
					 System.out.println("A");
				 }else if(sum >= 65 && sum <= 80){
					 System.out.println("B");
				 }else if(sum >= 50 && sum <= 65){
					 System.out.println("C");
				 }else if(sum >= 30 && sum <= 50){
					 if(r >= 50){
						 System.out.println("C");
					 }else{
						 System.out.println("D");
					 }
				 }else{
					 System.out.println("F");
				 }//if
			 }//if
		 }//while 
	} main {
		Scanner scan = new Scanner(System.in);
		int m = 0;	//中間試験
		int f = 0;	//期末試験
		int r = 0;	//再試験
		int sum;//中間試験と期末試験の点数の合計
        while(true){
			 m = Integer.parseInt(scan.next());
			 f = Integer.parseInt(scan.next());
			 r = Integer.parseInt(scan.next());
			 sum = m + f;
			 if(m == -1 && f ==-1 && r == -1){
				 break;
			 }else{
				 if(m == -1 || f == -1){
					 System.out.println("F");
				 }else if(sum >= 80){
					 System.out.println("A");
				 }else if(sum >= 65 && sum <= 80){
					 System.out.println("B");
				 }else if(sum >= 50 && sum <= 65){
					 System.out.println("C");
				 }else if(sum >= 30 && sum <= 50){
					 if(r >= 50){
						 System.out.println("C");
					 }else{
						 System.out.println("D");
					 }
				 }else{
					 System.out.println("F");
				 }//if
			 }//if
		 }//while 
	} Scanner scan = new Scanner(System.in); scan new Scanner(System.in) int m = 0; m 0 int f = 0; f 0 int r = 0; r 0 int sum; sum while(true){
			 m = Integer.parseInt(scan.next());
			 f = Integer.parseInt(scan.next());
			 r = Integer.parseInt(scan.next());
			 sum = m + f;
			 if(m == -1 && f ==-1 && r == -1){
				 break;
			 }else{
				 if(m == -1 || f == -1){
					 System.out.println("F");
				 }else if(sum >= 80){
					 System.out.println("A");
				 }else if(sum >= 65 && sum <= 80){
					 System.out.println("B");
				 }else if(sum >= 50 && sum <= 65){
					 System.out.println("C");
				 }else if(sum >= 30 && sum <= 50){
					 if(r >= 50){
						 System.out.println("C");
					 }else{
						 System.out.println("D");
					 }
				 }else{
					 System.out.println("F");
				 }//if
			 }//if
		 }//while  true {
			 m = Integer.parseInt(scan.next());
			 f = Integer.parseInt(scan.next());
			 r = Integer.parseInt(scan.next());
			 sum = m + f;
			 if(m == -1 && f ==-1 && r == -1){
				 break;
			 }else{
				 if(m == -1 || f == -1){
					 System.out.println("F");
				 }else if(sum >= 80){
					 System.out.println("A");
				 }else if(sum >= 65 && sum <= 80){
					 System.out.println("B");
				 }else if(sum >= 50 && sum <= 65){
					 System.out.println("C");
				 }else if(sum >= 30 && sum <= 50){
					 if(r >= 50){
						 System.out.println("C");
					 }else{
						 System.out.println("D");
					 }
				 }else{
					 System.out.println("F");
				 }//if
			 }//if
		 } m = Integer.parseInt(scan.next()) m Integer.parseInt(scan.next()) Integer.parseInt Integer scan.next() scan.next scan f = Integer.parseInt(scan.next()) f Integer.parseInt(scan.next()) Integer.parseInt Integer scan.next() scan.next scan r = Integer.parseInt(scan.next()) r Integer.parseInt(scan.next()) Integer.parseInt Integer scan.next() scan.next scan sum = m + f sum m + f m f if(m == -1 && f ==-1 && r == -1){
				 break;
			 }else{
				 if(m == -1 || f == -1){
					 System.out.println("F");
				 }else if(sum >= 80){
					 System.out.println("A");
				 }else if(sum >= 65 && sum <= 80){
					 System.out.println("B");
				 }else if(sum >= 50 && sum <= 65){
					 System.out.println("C");
				 }else if(sum >= 30 && sum <= 50){
					 if(r >= 50){
						 System.out.println("C");
					 }else{
						 System.out.println("D");
					 }
				 }else{
					 System.out.println("F");
				 }//if
			 }//if 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(sum >= 80){
					 System.out.println("A");
				 }else if(sum >= 65 && sum <= 80){
					 System.out.println("B");
				 }else if(sum >= 50 && sum <= 65){
					 System.out.println("C");
				 }else if(sum >= 30 && sum <= 50){
					 if(r >= 50){
						 System.out.println("C");
					 }else{
						 System.out.println("D");
					 }
				 }else{
					 System.out.println("F");
				 }//if
			 } if(m == -1 || f == -1){
					 System.out.println("F");
				 }else if(sum >= 80){
					 System.out.println("A");
				 }else if(sum >= 65 && sum <= 80){
					 System.out.println("B");
				 }else if(sum >= 50 && sum <= 65){
					 System.out.println("C");
				 }else if(sum >= 30 && sum <= 50){
					 if(r >= 50){
						 System.out.println("C");
					 }else{
						 System.out.println("D");
					 }
				 }else{
					 System.out.println("F");
				 }//if 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(sum >= 80){
					 System.out.println("A");
				 }else if(sum >= 65 && sum <= 80){
					 System.out.println("B");
				 }else if(sum >= 50 && sum <= 65){
					 System.out.println("C");
				 }else if(sum >= 30 && sum <= 50){
					 if(r >= 50){
						 System.out.println("C");
					 }else{
						 System.out.println("D");
					 }
				 }else{
					 System.out.println("F");
				 }//if sum >= 80 sum 80 {
					 System.out.println("A");
				 } System.out.println("A") System.out.println System.out System System.out "A" if(sum >= 65 && sum <= 80){
					 System.out.println("B");
				 }else if(sum >= 50 && sum <= 65){
					 System.out.println("C");
				 }else if(sum >= 30 && sum <= 50){
					 if(r >= 50){
						 System.out.println("C");
					 }else{
						 System.out.println("D");
					 }
				 }else{
					 System.out.println("F");
				 }//if sum >= 65 && sum <= 80 sum >= 65 sum 65 sum <= 80 sum 80 {
					 System.out.println("B");
				 } System.out.println("B") System.out.println System.out System System.out "B" if(sum >= 50 && sum <= 65){
					 System.out.println("C");
				 }else if(sum >= 30 && sum <= 50){
					 if(r >= 50){
						 System.out.println("C");
					 }else{
						 System.out.println("D");
					 }
				 }else{
					 System.out.println("F");
				 }//if sum >= 50 && sum <= 65 sum >= 50 sum 50 sum <= 65 sum 65 {
					 System.out.println("C");
				 } System.out.println("C") System.out.println System.out System System.out "C" if(sum >= 30 && sum <= 50){
					 if(r >= 50){
						 System.out.println("C");
					 }else{
						 System.out.println("D");
					 }
				 }else{
					 System.out.println("F");
				 }//if sum >= 30 && sum <= 50 sum >= 30 sum 30 sum <= 50 sum 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" {
					 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) {
		Scanner scan = new Scanner(System.in);
		int m = 0;	//中間試験
		int f = 0;	//期末試験
		int r = 0;	//再試験
		int sum;//中間試験と期末試験の点数の合計
        while(true){
			 m = Integer.parseInt(scan.next());
			 f = Integer.parseInt(scan.next());
			 r = Integer.parseInt(scan.next());
			 sum = m + f;
			 if(m == -1 && f ==-1 && r == -1){
				 break;
			 }else{
				 if(m == -1 || f == -1){
					 System.out.println("F");
				 }else if(sum >= 80){
					 System.out.println("A");
				 }else if(sum >= 65 && sum <= 80){
					 System.out.println("B");
				 }else if(sum >= 50 && sum <= 65){
					 System.out.println("C");
				 }else if(sum >= 30 && sum <= 50){
					 if(r >= 50){
						 System.out.println("C");
					 }else{
						 System.out.println("D");
					 }
				 }else{
					 System.out.println("F");
				 }//if
			 }//if
		 }//while 
	}
} public class Main {
	public static void main(String[] args) {
		Scanner scan = new Scanner(System.in);
		int m = 0;	//中間試験
		int f = 0;	//期末試験
		int r = 0;	//再試験
		int sum;//中間試験と期末試験の点数の合計
        while(true){
			 m = Integer.parseInt(scan.next());
			 f = Integer.parseInt(scan.next());
			 r = Integer.parseInt(scan.next());
			 sum = m + f;
			 if(m == -1 && f ==-1 && r == -1){
				 break;
			 }else{
				 if(m == -1 || f == -1){
					 System.out.println("F");
				 }else if(sum >= 80){
					 System.out.println("A");
				 }else if(sum >= 65 && sum <= 80){
					 System.out.println("B");
				 }else if(sum >= 50 && sum <= 65){
					 System.out.println("C");
				 }else if(sum >= 30 && sum <= 50){
					 if(r >= 50){
						 System.out.println("C");
					 }else{
						 System.out.println("D");
					 }
				 }else{
					 System.out.println("F");
				 }//if
			 }//if
		 }//while 
	}
} Main
import java.util.*;
public class Main {
	public static void main(String[] args) {
		Scanner scan = new Scanner(System.in);
		int m = 0;	//中間試験
		int f = 0;	//期末試験
		int r = 0;	//再試験
		int sum;//中間試験と期末試験の点数の合計
        while(true){
			 m = Integer.parseInt(scan.next());
			 f = Integer.parseInt(scan.next());
			 r = Integer.parseInt(scan.next());
			 sum = m + f;
			 if(m == -1 && f ==-1 && r == -1){
				 break;
			 }else{
				 if(m == -1 || f == -1){
					 System.out.println("F");
				 }else if(sum >= 80){
					 System.out.println("A");
				 }else if(sum >= 65 && sum <= 80){
					 System.out.println("B");
				 }else if(sum >= 50 && sum <= 65){
					 System.out.println("C");
				 }else if(sum >= 30 && sum <= 50){
					 if(r >= 50){
						 System.out.println("C");
					 }else{
						 System.out.println("D");
					 }
				 }else{
					 System.out.println("F");
				 }//if
			 }//if
		 }//while 
	}
}