import java.util.Scanner;

class Main {
    public static void main(String[] args) {
        Scanner scan = new Scanner(System.in);

        int m;
        int f;
        int r;

        while(true){

        	m = scan.nextInt();
        	f = scan.nextInt();
        	r = scan.nextInt();
        	int sum = m+f;

        	if( m == -1 && f == -1 && r == -1){
        		break;
        	}
        	else if(m == -1 || f == -1){
        		System.out.println("F");
        	}else if(sum < 30 ){
        		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 && r == -1){
        		System.out.println("C");
        	}
        	else if(sum >= 50 && sum < 65 && r >= 50){
        		System.out.println("C");
        	}
        	else if(sum >= 30 && sum < 50 && r >= 50){
        		System.out.println("C");
        	}
        	else if(sum >= 30 && sum < 50 && r < 50){
        		System.out.println("D");
        	}
        	else if(sum >= 30 && sum < 50 && m == 0 && r < 50){
        		System.out.println("D");
        	}
        	else if(sum >= 30 && sum < 50 && f == 0 && r < 50){
        		System.out.println("D");
        	}
        }

     }
} import java.util.Scanner; Scanner java.util class Main {
    public static void main(String[] args) {
        Scanner scan = new Scanner(System.in);

        int m;
        int f;
        int r;

        while(true){

        	m = scan.nextInt();
        	f = scan.nextInt();
        	r = scan.nextInt();
        	int sum = m+f;

        	if( m == -1 && f == -1 && r == -1){
        		break;
        	}
        	else if(m == -1 || f == -1){
        		System.out.println("F");
        	}else if(sum < 30 ){
        		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 && r == -1){
        		System.out.println("C");
        	}
        	else if(sum >= 50 && sum < 65 && r >= 50){
        		System.out.println("C");
        	}
        	else if(sum >= 30 && sum < 50 && r >= 50){
        		System.out.println("C");
        	}
        	else if(sum >= 30 && sum < 50 && r < 50){
        		System.out.println("D");
        	}
        	else if(sum >= 30 && sum < 50 && m == 0 && r < 50){
        		System.out.println("D");
        	}
        	else if(sum >= 30 && sum < 50 && f == 0 && r < 50){
        		System.out.println("D");
        	}
        }

     }
} Main public static void main(String[] args) {
        Scanner scan = new Scanner(System.in);

        int m;
        int f;
        int r;

        while(true){

        	m = scan.nextInt();
        	f = scan.nextInt();
        	r = scan.nextInt();
        	int sum = m+f;

        	if( m == -1 && f == -1 && r == -1){
        		break;
        	}
        	else if(m == -1 || f == -1){
        		System.out.println("F");
        	}else if(sum < 30 ){
        		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 && r == -1){
        		System.out.println("C");
        	}
        	else if(sum >= 50 && sum < 65 && r >= 50){
        		System.out.println("C");
        	}
        	else if(sum >= 30 && sum < 50 && r >= 50){
        		System.out.println("C");
        	}
        	else if(sum >= 30 && sum < 50 && r < 50){
        		System.out.println("D");
        	}
        	else if(sum >= 30 && sum < 50 && m == 0 && r < 50){
        		System.out.println("D");
        	}
        	else if(sum >= 30 && sum < 50 && f == 0 && r < 50){
        		System.out.println("D");
        	}
        }

     } main {
        Scanner scan = new Scanner(System.in);

        int m;
        int f;
        int r;

        while(true){

        	m = scan.nextInt();
        	f = scan.nextInt();
        	r = scan.nextInt();
        	int sum = m+f;

        	if( m == -1 && f == -1 && r == -1){
        		break;
        	}
        	else if(m == -1 || f == -1){
        		System.out.println("F");
        	}else if(sum < 30 ){
        		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 && r == -1){
        		System.out.println("C");
        	}
        	else if(sum >= 50 && sum < 65 && r >= 50){
        		System.out.println("C");
        	}
        	else if(sum >= 30 && sum < 50 && r >= 50){
        		System.out.println("C");
        	}
        	else if(sum >= 30 && sum < 50 && r < 50){
        		System.out.println("D");
        	}
        	else if(sum >= 30 && sum < 50 && m == 0 && r < 50){
        		System.out.println("D");
        	}
        	else if(sum >= 30 && sum < 50 && f == 0 && r < 50){
        		System.out.println("D");
        	}
        }

     } Scanner scan = new Scanner(System.in); scan new Scanner(System.in) int m; m int f; f int r; r while(true){

        	m = scan.nextInt();
        	f = scan.nextInt();
        	r = scan.nextInt();
        	int sum = m+f;

        	if( m == -1 && f == -1 && r == -1){
        		break;
        	}
        	else if(m == -1 || f == -1){
        		System.out.println("F");
        	}else if(sum < 30 ){
        		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 && r == -1){
        		System.out.println("C");
        	}
        	else if(sum >= 50 && sum < 65 && r >= 50){
        		System.out.println("C");
        	}
        	else if(sum >= 30 && sum < 50 && r >= 50){
        		System.out.println("C");
        	}
        	else if(sum >= 30 && sum < 50 && r < 50){
        		System.out.println("D");
        	}
        	else if(sum >= 30 && sum < 50 && m == 0 && r < 50){
        		System.out.println("D");
        	}
        	else if(sum >= 30 && sum < 50 && f == 0 && r < 50){
        		System.out.println("D");
        	}
        } true {

        	m = scan.nextInt();
        	f = scan.nextInt();
        	r = scan.nextInt();
        	int sum = m+f;

        	if( m == -1 && f == -1 && r == -1){
        		break;
        	}
        	else if(m == -1 || f == -1){
        		System.out.println("F");
        	}else if(sum < 30 ){
        		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 && r == -1){
        		System.out.println("C");
        	}
        	else if(sum >= 50 && sum < 65 && r >= 50){
        		System.out.println("C");
        	}
        	else if(sum >= 30 && sum < 50 && r >= 50){
        		System.out.println("C");
        	}
        	else if(sum >= 30 && sum < 50 && r < 50){
        		System.out.println("D");
        	}
        	else if(sum >= 30 && sum < 50 && m == 0 && r < 50){
        		System.out.println("D");
        	}
        	else if(sum >= 30 && sum < 50 && f == 0 && r < 50){
        		System.out.println("D");
        	}
        } m = scan.nextInt() m scan.nextInt() scan.nextInt scan f = scan.nextInt() f scan.nextInt() scan.nextInt scan r = scan.nextInt() r scan.nextInt() scan.nextInt scan int 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 < 30 ){
        		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 && r == -1){
        		System.out.println("C");
        	}
        	else if(sum >= 50 && sum < 65 && r >= 50){
        		System.out.println("C");
        	}
        	else if(sum >= 30 && sum < 50 && r >= 50){
        		System.out.println("C");
        	}
        	else if(sum >= 30 && sum < 50 && r < 50){
        		System.out.println("D");
        	}
        	else if(sum >= 30 && sum < 50 && m == 0 && r < 50){
        		System.out.println("D");
        	}
        	else if(sum >= 30 && sum < 50 && f == 0 && r < 50){
        		System.out.println("D");
        	} 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 < 30 ){
        		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 && r == -1){
        		System.out.println("C");
        	}
        	else if(sum >= 50 && sum < 65 && r >= 50){
        		System.out.println("C");
        	}
        	else if(sum >= 30 && sum < 50 && r >= 50){
        		System.out.println("C");
        	}
        	else if(sum >= 30 && sum < 50 && r < 50){
        		System.out.println("D");
        	}
        	else if(sum >= 30 && sum < 50 && m == 0 && r < 50){
        		System.out.println("D");
        	}
        	else if(sum >= 30 && sum < 50 && f == 0 && r < 50){
        		System.out.println("D");
        	} 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 < 30 ){
        		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 && r == -1){
        		System.out.println("C");
        	}
        	else if(sum >= 50 && sum < 65 && r >= 50){
        		System.out.println("C");
        	}
        	else if(sum >= 30 && sum < 50 && r >= 50){
        		System.out.println("C");
        	}
        	else if(sum >= 30 && sum < 50 && r < 50){
        		System.out.println("D");
        	}
        	else if(sum >= 30 && sum < 50 && m == 0 && r < 50){
        		System.out.println("D");
        	}
        	else if(sum >= 30 && sum < 50 && f == 0 && r < 50){
        		System.out.println("D");
        	} sum < 30 sum 30 {
        		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 && r == -1){
        		System.out.println("C");
        	}
        	else if(sum >= 50 && sum < 65 && r >= 50){
        		System.out.println("C");
        	}
        	else if(sum >= 30 && sum < 50 && r >= 50){
        		System.out.println("C");
        	}
        	else if(sum >= 30 && sum < 50 && r < 50){
        		System.out.println("D");
        	}
        	else if(sum >= 30 && sum < 50 && m == 0 && r < 50){
        		System.out.println("D");
        	}
        	else if(sum >= 30 && sum < 50 && f == 0 && r < 50){
        		System.out.println("D");
        	} 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 && r == -1){
        		System.out.println("C");
        	}
        	else if(sum >= 50 && sum < 65 && r >= 50){
        		System.out.println("C");
        	}
        	else if(sum >= 30 && sum < 50 && r >= 50){
        		System.out.println("C");
        	}
        	else if(sum >= 30 && sum < 50 && r < 50){
        		System.out.println("D");
        	}
        	else if(sum >= 30 && sum < 50 && m == 0 && r < 50){
        		System.out.println("D");
        	}
        	else if(sum >= 30 && sum < 50 && f == 0 && r < 50){
        		System.out.println("D");
        	} 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 && r == -1){
        		System.out.println("C");
        	}
        	else if(sum >= 50 && sum < 65 && r >= 50){
        		System.out.println("C");
        	}
        	else if(sum >= 30 && sum < 50 && r >= 50){
        		System.out.println("C");
        	}
        	else if(sum >= 30 && sum < 50 && r < 50){
        		System.out.println("D");
        	}
        	else if(sum >= 30 && sum < 50 && m == 0 && r < 50){
        		System.out.println("D");
        	}
        	else if(sum >= 30 && sum < 50 && f == 0 && r < 50){
        		System.out.println("D");
        	} sum >= 50 && sum < 65 && r == -1 sum >= 50 && sum < 65 && r == -1 sum >= 50 sum 50 sum < 65 sum 65 r == -1 r -1 1 {
        		System.out.println("C");
        	} System.out.println("C") System.out.println System.out System System.out "C" if(sum >= 50 && sum < 65 && r >= 50){
        		System.out.println("C");
        	}
        	else if(sum >= 30 && sum < 50 && r >= 50){
        		System.out.println("C");
        	}
        	else if(sum >= 30 && sum < 50 && r < 50){
        		System.out.println("D");
        	}
        	else if(sum >= 30 && sum < 50 && m == 0 && r < 50){
        		System.out.println("D");
        	}
        	else if(sum >= 30 && sum < 50 && f == 0 && r < 50){
        		System.out.println("D");
        	} sum >= 50 && sum < 65 && r >= 50 sum >= 50 && sum < 65 && r >= 50 sum >= 50 sum 50 sum < 65 sum 65 r >= 50 r 50 {
        		System.out.println("C");
        	} System.out.println("C") System.out.println System.out System System.out "C" if(sum >= 30 && sum < 50 && r >= 50){
        		System.out.println("C");
        	}
        	else if(sum >= 30 && sum < 50 && r < 50){
        		System.out.println("D");
        	}
        	else if(sum >= 30 && sum < 50 && m == 0 && r < 50){
        		System.out.println("D");
        	}
        	else if(sum >= 30 && sum < 50 && f == 0 && r < 50){
        		System.out.println("D");
        	} sum >= 30 && sum < 50 && r >= 50 sum >= 30 && sum < 50 && r >= 50 sum >= 30 sum 30 sum < 50 sum 50 r >= 50 r 50 {
        		System.out.println("C");
        	} System.out.println("C") System.out.println System.out System System.out "C" if(sum >= 30 && sum < 50 && r < 50){
        		System.out.println("D");
        	}
        	else if(sum >= 30 && sum < 50 && m == 0 && r < 50){
        		System.out.println("D");
        	}
        	else if(sum >= 30 && sum < 50 && f == 0 && r < 50){
        		System.out.println("D");
        	} sum >= 30 && sum < 50 && r < 50 sum >= 30 && sum < 50 && r < 50 sum >= 30 sum 30 sum < 50 sum 50 r < 50 r 50 {
        		System.out.println("D");
        	} System.out.println("D") System.out.println System.out System System.out "D" if(sum >= 30 && sum < 50 && m == 0 && r < 50){
        		System.out.println("D");
        	}
        	else if(sum >= 30 && sum < 50 && f == 0 && r < 50){
        		System.out.println("D");
        	} sum >= 30 && sum < 50 && m == 0 && r < 50 m == 0 sum >= 30 && sum < 50 && m == 0 && r < 50 sum >= 30 sum 30 sum < 50 sum 50 m == 0 m 0 r < 50 r 50 {
        		System.out.println("D");
        	} System.out.println("D") System.out.println System.out System System.out "D" if(sum >= 30 && sum < 50 && f == 0 && r < 50){
        		System.out.println("D");
        	} sum >= 30 && sum < 50 && f == 0 && r < 50 f == 0 sum >= 30 && sum < 50 && f == 0 && r < 50 sum >= 30 sum 30 sum < 50 sum 50 f == 0 f 0 r < 50 r 50 {
        		System.out.println("D");
        	} System.out.println("D") System.out.println System.out System System.out "D" String[] args args
import java.util.Scanner;

class Main {
    public static void main(String[] args) {
        Scanner scan = new Scanner(System.in);

        int m;
        int f;
        int r;

        while(true){

        	m = scan.nextInt();
        	f = scan.nextInt();
        	r = scan.nextInt();
        	int sum = m+f;

        	if( m == -1 && f == -1 && r == -1){
        		break;
        	}
        	else if(m == -1 || f == -1){
        		System.out.println("F");
        	}else if(sum < 30 ){
        		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 && r == -1){
        		System.out.println("C");
        	}
        	else if(sum >= 50 && sum < 65 && r >= 50){
        		System.out.println("C");
        	}
        	else if(sum >= 30 && sum < 50 && r >= 50){
        		System.out.println("C");
        	}
        	else if(sum >= 30 && sum < 50 && r < 50){
        		System.out.println("D");
        	}
        	else if(sum >= 30 && sum < 50 && m == 0 && r < 50){
        		System.out.println("D");
        	}
        	else if(sum >= 30 && sum < 50 && f == 0 && r < 50){
        		System.out.println("D");
        	}
        }

     }
}