import java.io.*;
public class Main {
	public static void main(String[] args) throws IOException{
		BufferedReader reader = new BufferedReader(new InputStreamReader(System.in));
		while(true){
			String line = reader.readLine();
			String[] sarray = line.split(" ", -1);
			int m = Integer.parseInt(sarray[0]);
			int f = Integer.parseInt(sarray[1]);
			int r = Integer.parseInt(sarray[2]);
			
			if(m == -1 || f == -1){
				if(m == -1 && f == -1){
					break;
				}else{
					System.out.println("F");
				}
			}else{
				if((m + f) >= 80){
				System.out.println("A");
				}
				if((m + f) >= 65 && (m + f) < 80){
					System.out.println("B");
				}
				if((m + f) >= 50 && (m + f) < 65){
					System.out.println("C");
				}
				if((m + f) >= 30 && (m + f) < 50){
					if(r >= 50){
						System.out.println("C");
					}else{
						System.out.println("D");
					}
				}
				if((m + f) >= 0 && (m + f) < 30){
					System.out.println("F");
				}
			}
		}
	}
} import java.io.*; io.* java public class Main {
	public static void main(String[] args) throws IOException{
		BufferedReader reader = new BufferedReader(new InputStreamReader(System.in));
		while(true){
			String line = reader.readLine();
			String[] sarray = line.split(" ", -1);
			int m = Integer.parseInt(sarray[0]);
			int f = Integer.parseInt(sarray[1]);
			int r = Integer.parseInt(sarray[2]);
			
			if(m == -1 || f == -1){
				if(m == -1 && f == -1){
					break;
				}else{
					System.out.println("F");
				}
			}else{
				if((m + f) >= 80){
				System.out.println("A");
				}
				if((m + f) >= 65 && (m + f) < 80){
					System.out.println("B");
				}
				if((m + f) >= 50 && (m + f) < 65){
					System.out.println("C");
				}
				if((m + f) >= 30 && (m + f) < 50){
					if(r >= 50){
						System.out.println("C");
					}else{
						System.out.println("D");
					}
				}
				if((m + f) >= 0 && (m + f) < 30){
					System.out.println("F");
				}
			}
		}
	}
} Main public static void main(String[] args) throws IOException{
		BufferedReader reader = new BufferedReader(new InputStreamReader(System.in));
		while(true){
			String line = reader.readLine();
			String[] sarray = line.split(" ", -1);
			int m = Integer.parseInt(sarray[0]);
			int f = Integer.parseInt(sarray[1]);
			int r = Integer.parseInt(sarray[2]);
			
			if(m == -1 || f == -1){
				if(m == -1 && f == -1){
					break;
				}else{
					System.out.println("F");
				}
			}else{
				if((m + f) >= 80){
				System.out.println("A");
				}
				if((m + f) >= 65 && (m + f) < 80){
					System.out.println("B");
				}
				if((m + f) >= 50 && (m + f) < 65){
					System.out.println("C");
				}
				if((m + f) >= 30 && (m + f) < 50){
					if(r >= 50){
						System.out.println("C");
					}else{
						System.out.println("D");
					}
				}
				if((m + f) >= 0 && (m + f) < 30){
					System.out.println("F");
				}
			}
		}
	} main {
		BufferedReader reader = new BufferedReader(new InputStreamReader(System.in));
		while(true){
			String line = reader.readLine();
			String[] sarray = line.split(" ", -1);
			int m = Integer.parseInt(sarray[0]);
			int f = Integer.parseInt(sarray[1]);
			int r = Integer.parseInt(sarray[2]);
			
			if(m == -1 || f == -1){
				if(m == -1 && f == -1){
					break;
				}else{
					System.out.println("F");
				}
			}else{
				if((m + f) >= 80){
				System.out.println("A");
				}
				if((m + f) >= 65 && (m + f) < 80){
					System.out.println("B");
				}
				if((m + f) >= 50 && (m + f) < 65){
					System.out.println("C");
				}
				if((m + f) >= 30 && (m + f) < 50){
					if(r >= 50){
						System.out.println("C");
					}else{
						System.out.println("D");
					}
				}
				if((m + f) >= 0 && (m + f) < 30){
					System.out.println("F");
				}
			}
		}
	} BufferedReader reader = new BufferedReader(new InputStreamReader(System.in)); reader new BufferedReader(new InputStreamReader(System.in)) while(true){
			String line = reader.readLine();
			String[] sarray = line.split(" ", -1);
			int m = Integer.parseInt(sarray[0]);
			int f = Integer.parseInt(sarray[1]);
			int r = Integer.parseInt(sarray[2]);
			
			if(m == -1 || f == -1){
				if(m == -1 && f == -1){
					break;
				}else{
					System.out.println("F");
				}
			}else{
				if((m + f) >= 80){
				System.out.println("A");
				}
				if((m + f) >= 65 && (m + f) < 80){
					System.out.println("B");
				}
				if((m + f) >= 50 && (m + f) < 65){
					System.out.println("C");
				}
				if((m + f) >= 30 && (m + f) < 50){
					if(r >= 50){
						System.out.println("C");
					}else{
						System.out.println("D");
					}
				}
				if((m + f) >= 0 && (m + f) < 30){
					System.out.println("F");
				}
			}
		} true {
			String line = reader.readLine();
			String[] sarray = line.split(" ", -1);
			int m = Integer.parseInt(sarray[0]);
			int f = Integer.parseInt(sarray[1]);
			int r = Integer.parseInt(sarray[2]);
			
			if(m == -1 || f == -1){
				if(m == -1 && f == -1){
					break;
				}else{
					System.out.println("F");
				}
			}else{
				if((m + f) >= 80){
				System.out.println("A");
				}
				if((m + f) >= 65 && (m + f) < 80){
					System.out.println("B");
				}
				if((m + f) >= 50 && (m + f) < 65){
					System.out.println("C");
				}
				if((m + f) >= 30 && (m + f) < 50){
					if(r >= 50){
						System.out.println("C");
					}else{
						System.out.println("D");
					}
				}
				if((m + f) >= 0 && (m + f) < 30){
					System.out.println("F");
				}
			}
		} String line = reader.readLine(); line reader.readLine() reader.readLine reader String[] sarray = line.split(" ", -1); sarray line.split(" ", -1) line.split line " " -1 1 int m = Integer.parseInt(sarray[0]); m Integer.parseInt(sarray[0]) Integer.parseInt Integer sarray[0] sarray 0 int f = Integer.parseInt(sarray[1]); f Integer.parseInt(sarray[1]) Integer.parseInt Integer sarray[1] sarray 1 int r = Integer.parseInt(sarray[2]); r Integer.parseInt(sarray[2]) Integer.parseInt Integer sarray[2] sarray 2 if(m == -1 || f == -1){
				if(m == -1 && f == -1){
					break;
				}else{
					System.out.println("F");
				}
			}else{
				if((m + f) >= 80){
				System.out.println("A");
				}
				if((m + f) >= 65 && (m + f) < 80){
					System.out.println("B");
				}
				if((m + f) >= 50 && (m + f) < 65){
					System.out.println("C");
				}
				if((m + f) >= 30 && (m + f) < 50){
					if(r >= 50){
						System.out.println("C");
					}else{
						System.out.println("D");
					}
				}
				if((m + f) >= 0 && (m + f) < 30){
					System.out.println("F");
				}
			} m == -1 || f == -1 m == -1 m -1 1 f == -1 f -1 1 {
				if(m == -1 && f == -1){
					break;
				}else{
					System.out.println("F");
				}
			} if(m == -1 && f == -1){
					break;
				}else{
					System.out.println("F");
				} m == -1 && f == -1 m == -1 m -1 1 f == -1 f -1 1 {
					break;
				} break; {
					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");
				}
				if((m + f) >= 65 && (m + f) < 80){
					System.out.println("B");
				}
				if((m + f) >= 50 && (m + f) < 65){
					System.out.println("C");
				}
				if((m + f) >= 30 && (m + f) < 50){
					if(r >= 50){
						System.out.println("C");
					}else{
						System.out.println("D");
					}
				}
				if((m + f) >= 0 && (m + f) < 30){
					System.out.println("F");
				}
			} if((m + f) >= 80){
				System.out.println("A");
				} (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");
				} (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");
				} (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" if((m + f) >= 0 && (m + f) < 30){
					System.out.println("F");
				} (m + f) >= 0 && (m + f) < 30 (m + f) >= 0 (m + f) m f 0 (m + f) < 30 (m + f) m f 30 {
					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 reader = new BufferedReader(new InputStreamReader(System.in));
		while(true){
			String line = reader.readLine();
			String[] sarray = line.split(" ", -1);
			int m = Integer.parseInt(sarray[0]);
			int f = Integer.parseInt(sarray[1]);
			int r = Integer.parseInt(sarray[2]);
			
			if(m == -1 || f == -1){
				if(m == -1 && f == -1){
					break;
				}else{
					System.out.println("F");
				}
			}else{
				if((m + f) >= 80){
				System.out.println("A");
				}
				if((m + f) >= 65 && (m + f) < 80){
					System.out.println("B");
				}
				if((m + f) >= 50 && (m + f) < 65){
					System.out.println("C");
				}
				if((m + f) >= 30 && (m + f) < 50){
					if(r >= 50){
						System.out.println("C");
					}else{
						System.out.println("D");
					}
				}
				if((m + f) >= 0 && (m + f) < 30){
					System.out.println("F");
				}
			}
		}
	}
} public class Main {
	public static void main(String[] args) throws IOException{
		BufferedReader reader = new BufferedReader(new InputStreamReader(System.in));
		while(true){
			String line = reader.readLine();
			String[] sarray = line.split(" ", -1);
			int m = Integer.parseInt(sarray[0]);
			int f = Integer.parseInt(sarray[1]);
			int r = Integer.parseInt(sarray[2]);
			
			if(m == -1 || f == -1){
				if(m == -1 && f == -1){
					break;
				}else{
					System.out.println("F");
				}
			}else{
				if((m + f) >= 80){
				System.out.println("A");
				}
				if((m + f) >= 65 && (m + f) < 80){
					System.out.println("B");
				}
				if((m + f) >= 50 && (m + f) < 65){
					System.out.println("C");
				}
				if((m + f) >= 30 && (m + f) < 50){
					if(r >= 50){
						System.out.println("C");
					}else{
						System.out.println("D");
					}
				}
				if((m + f) >= 0 && (m + f) < 30){
					System.out.println("F");
				}
			}
		}
	}
} Main
import java.io.*;
public class Main {
	public static void main(String[] args) throws IOException{
		BufferedReader reader = new BufferedReader(new InputStreamReader(System.in));
		while(true){
			String line = reader.readLine();
			String[] sarray = line.split(" ", -1);
			int m = Integer.parseInt(sarray[0]);
			int f = Integer.parseInt(sarray[1]);
			int r = Integer.parseInt(sarray[2]);
			
			if(m == -1 || f == -1){
				if(m == -1 && f == -1){
					break;
				}else{
					System.out.println("F");
				}
			}else{
				if((m + f) >= 80){
				System.out.println("A");
				}
				if((m + f) >= 65 && (m + f) < 80){
					System.out.println("B");
				}
				if((m + f) >= 50 && (m + f) < 65){
					System.out.println("C");
				}
				if((m + f) >= 30 && (m + f) < 50){
					if(r >= 50){
						System.out.println("C");
					}else{
						System.out.println("D");
					}
				}
				if((m + f) >= 0 && (m + f) < 30){
					System.out.println("F");
				}
			}
		}
	}
}