import java.util.Scanner;

public class Main{
	public static void main(String[] args){
		Scanner scan = null;
		try{
			scan = new Scanner(System.in);
			int m,f,r;
			while(true){
				m = scan.nextInt();
				f = scan.nextInt();
				r = scan.nextInt();
				if((m == f) &&(f == r) &&(m == -1))break;
				int sum = m + f;
				if(m == -1 || f == -1){
					System.out.println("F");
				}else if(sum >= 80){
					System.out.println("A");
				}else if(sum >=65){
					System.out.println("B");
				}else if(sum >=50){
					System.out.println("C");
				}else if(sum >=30){
					if(r >=50){
						System.out.println("C");
					}else{
						System.out.println("D");
					}
				}else{
					System.out.println("F");
				}
			}
		}finally{
			if(scan !=null)scan.close();
		}
	}
} import java.util.Scanner; Scanner java.util public class Main{
	public static void main(String[] args){
		Scanner scan = null;
		try{
			scan = new Scanner(System.in);
			int m,f,r;
			while(true){
				m = scan.nextInt();
				f = scan.nextInt();
				r = scan.nextInt();
				if((m == f) &&(f == r) &&(m == -1))break;
				int sum = m + f;
				if(m == -1 || f == -1){
					System.out.println("F");
				}else if(sum >= 80){
					System.out.println("A");
				}else if(sum >=65){
					System.out.println("B");
				}else if(sum >=50){
					System.out.println("C");
				}else if(sum >=30){
					if(r >=50){
						System.out.println("C");
					}else{
						System.out.println("D");
					}
				}else{
					System.out.println("F");
				}
			}
		}finally{
			if(scan !=null)scan.close();
		}
	}
} Main public static void main(String[] args){
		Scanner scan = null;
		try{
			scan = new Scanner(System.in);
			int m,f,r;
			while(true){
				m = scan.nextInt();
				f = scan.nextInt();
				r = scan.nextInt();
				if((m == f) &&(f == r) &&(m == -1))break;
				int sum = m + f;
				if(m == -1 || f == -1){
					System.out.println("F");
				}else if(sum >= 80){
					System.out.println("A");
				}else if(sum >=65){
					System.out.println("B");
				}else if(sum >=50){
					System.out.println("C");
				}else if(sum >=30){
					if(r >=50){
						System.out.println("C");
					}else{
						System.out.println("D");
					}
				}else{
					System.out.println("F");
				}
			}
		}finally{
			if(scan !=null)scan.close();
		}
	} main {
		Scanner scan = null;
		try{
			scan = new Scanner(System.in);
			int m,f,r;
			while(true){
				m = scan.nextInt();
				f = scan.nextInt();
				r = scan.nextInt();
				if((m == f) &&(f == r) &&(m == -1))break;
				int sum = m + f;
				if(m == -1 || f == -1){
					System.out.println("F");
				}else if(sum >= 80){
					System.out.println("A");
				}else if(sum >=65){
					System.out.println("B");
				}else if(sum >=50){
					System.out.println("C");
				}else if(sum >=30){
					if(r >=50){
						System.out.println("C");
					}else{
						System.out.println("D");
					}
				}else{
					System.out.println("F");
				}
			}
		}finally{
			if(scan !=null)scan.close();
		}
	} Scanner scan = null; scan null try{
			scan = new Scanner(System.in);
			int m,f,r;
			while(true){
				m = scan.nextInt();
				f = scan.nextInt();
				r = scan.nextInt();
				if((m == f) &&(f == r) &&(m == -1))break;
				int sum = m + f;
				if(m == -1 || f == -1){
					System.out.println("F");
				}else if(sum >= 80){
					System.out.println("A");
				}else if(sum >=65){
					System.out.println("B");
				}else if(sum >=50){
					System.out.println("C");
				}else if(sum >=30){
					if(r >=50){
						System.out.println("C");
					}else{
						System.out.println("D");
					}
				}else{
					System.out.println("F");
				}
			}
		}finally{
			if(scan !=null)scan.close();
		} {
			if(scan !=null)scan.close();
		} if(scan !=null)scan.close(); scan !=null scan null scan.close() scan.close scan {
			scan = new Scanner(System.in);
			int m,f,r;
			while(true){
				m = scan.nextInt();
				f = scan.nextInt();
				r = scan.nextInt();
				if((m == f) &&(f == r) &&(m == -1))break;
				int sum = m + f;
				if(m == -1 || f == -1){
					System.out.println("F");
				}else if(sum >= 80){
					System.out.println("A");
				}else if(sum >=65){
					System.out.println("B");
				}else if(sum >=50){
					System.out.println("C");
				}else if(sum >=30){
					if(r >=50){
						System.out.println("C");
					}else{
						System.out.println("D");
					}
				}else{
					System.out.println("F");
				}
			}
		} scan = new Scanner(System.in) scan new Scanner(System.in) int m m f f r; r while(true){
				m = scan.nextInt();
				f = scan.nextInt();
				r = scan.nextInt();
				if((m == f) &&(f == r) &&(m == -1))break;
				int sum = m + f;
				if(m == -1 || f == -1){
					System.out.println("F");
				}else if(sum >= 80){
					System.out.println("A");
				}else if(sum >=65){
					System.out.println("B");
				}else if(sum >=50){
					System.out.println("C");
				}else if(sum >=30){
					if(r >=50){
						System.out.println("C");
					}else{
						System.out.println("D");
					}
				}else{
					System.out.println("F");
				}
			} true {
				m = scan.nextInt();
				f = scan.nextInt();
				r = scan.nextInt();
				if((m == f) &&(f == r) &&(m == -1))break;
				int sum = m + f;
				if(m == -1 || f == -1){
					System.out.println("F");
				}else if(sum >= 80){
					System.out.println("A");
				}else if(sum >=65){
					System.out.println("B");
				}else if(sum >=50){
					System.out.println("C");
				}else if(sum >=30){
					if(r >=50){
						System.out.println("C");
					}else{
						System.out.println("D");
					}
				}else{
					System.out.println("F");
				}
			} 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 if((m == f) &&(f == r) &&(m == -1))break; (m == f) &&(f == r) &&(m == -1) (m == f) &&(f == r) &&(m == -1) (m == f) m f (f == r) f r (m == -1) m -1 1 break; int sum = m + f; sum m + f m f if(m == -1 || f == -1){
					System.out.println("F");
				}else if(sum >= 80){
					System.out.println("A");
				}else if(sum >=65){
					System.out.println("B");
				}else if(sum >=50){
					System.out.println("C");
				}else if(sum >=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(sum >= 80){
					System.out.println("A");
				}else if(sum >=65){
					System.out.println("B");
				}else if(sum >=50){
					System.out.println("C");
				}else if(sum >=30){
					if(r >=50){
						System.out.println("C");
					}else{
						System.out.println("D");
					}
				}else{
					System.out.println("F");
				} sum >= 80 sum 80 {
					System.out.println("A");
				} System.out.println("A") System.out.println System.out System System.out "A" if(sum >=65){
					System.out.println("B");
				}else if(sum >=50){
					System.out.println("C");
				}else if(sum >=30){
					if(r >=50){
						System.out.println("C");
					}else{
						System.out.println("D");
					}
				}else{
					System.out.println("F");
				} sum >=65 sum 65 {
					System.out.println("B");
				} System.out.println("B") System.out.println System.out System System.out "B" if(sum >=50){
					System.out.println("C");
				}else if(sum >=30){
					if(r >=50){
						System.out.println("C");
					}else{
						System.out.println("D");
					}
				}else{
					System.out.println("F");
				} sum >=50 sum 50 {
					System.out.println("C");
				} System.out.println("C") System.out.println System.out System System.out "C" if(sum >=30){
					if(r >=50){
						System.out.println("C");
					}else{
						System.out.println("D");
					}
				}else{
					System.out.println("F");
				} sum >=30 sum 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){
		Scanner scan = null;
		try{
			scan = new Scanner(System.in);
			int m,f,r;
			while(true){
				m = scan.nextInt();
				f = scan.nextInt();
				r = scan.nextInt();
				if((m == f) &&(f == r) &&(m == -1))break;
				int sum = m + f;
				if(m == -1 || f == -1){
					System.out.println("F");
				}else if(sum >= 80){
					System.out.println("A");
				}else if(sum >=65){
					System.out.println("B");
				}else if(sum >=50){
					System.out.println("C");
				}else if(sum >=30){
					if(r >=50){
						System.out.println("C");
					}else{
						System.out.println("D");
					}
				}else{
					System.out.println("F");
				}
			}
		}finally{
			if(scan !=null)scan.close();
		}
	}
} public class Main{
	public static void main(String[] args){
		Scanner scan = null;
		try{
			scan = new Scanner(System.in);
			int m,f,r;
			while(true){
				m = scan.nextInt();
				f = scan.nextInt();
				r = scan.nextInt();
				if((m == f) &&(f == r) &&(m == -1))break;
				int sum = m + f;
				if(m == -1 || f == -1){
					System.out.println("F");
				}else if(sum >= 80){
					System.out.println("A");
				}else if(sum >=65){
					System.out.println("B");
				}else if(sum >=50){
					System.out.println("C");
				}else if(sum >=30){
					if(r >=50){
						System.out.println("C");
					}else{
						System.out.println("D");
					}
				}else{
					System.out.println("F");
				}
			}
		}finally{
			if(scan !=null)scan.close();
		}
	}
} Main
import java.util.Scanner;

public class Main{
	public static void main(String[] args){
		Scanner scan = null;
		try{
			scan = new Scanner(System.in);
			int m,f,r;
			while(true){
				m = scan.nextInt();
				f = scan.nextInt();
				r = scan.nextInt();
				if((m == f) &&(f == r) &&(m == -1))break;
				int sum = m + f;
				if(m == -1 || f == -1){
					System.out.println("F");
				}else if(sum >= 80){
					System.out.println("A");
				}else if(sum >=65){
					System.out.println("B");
				}else if(sum >=50){
					System.out.println("C");
				}else if(sum >=30){
					if(r >=50){
						System.out.println("C");
					}else{
						System.out.println("D");
					}
				}else{
					System.out.println("F");
				}
			}
		}finally{
			if(scan !=null)scan.close();
		}
	}
}