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

public class Main {

	public static void main(String[] args) throws IOException {
		// TODO Auto-generated method stub
		String line;
		BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
		while ((line = br.readLine()) != null) {
			if (line.equals("-1 -1 -1")) {
				break;
			}

			String[] token = line.split(" ");
			int m, f, r, sum;
			m = Integer.parseInt(token[0]);
			f = Integer.parseInt(token[1]);
			r = Integer.parseInt(token[2]);
			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");
			}
		}
	}
} 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 static void main(String[] args) throws IOException {
		// TODO Auto-generated method stub
		String line;
		BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
		while ((line = br.readLine()) != null) {
			if (line.equals("-1 -1 -1")) {
				break;
			}

			String[] token = line.split(" ");
			int m, f, r, sum;
			m = Integer.parseInt(token[0]);
			f = Integer.parseInt(token[1]);
			r = Integer.parseInt(token[2]);
			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");
			}
		}
	}
} Main public static void main(String[] args) throws IOException {
		// TODO Auto-generated method stub
		String line;
		BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
		while ((line = br.readLine()) != null) {
			if (line.equals("-1 -1 -1")) {
				break;
			}

			String[] token = line.split(" ");
			int m, f, r, sum;
			m = Integer.parseInt(token[0]);
			f = Integer.parseInt(token[1]);
			r = Integer.parseInt(token[2]);
			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");
			}
		}
	} main {
		// TODO Auto-generated method stub
		String line;
		BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
		while ((line = br.readLine()) != null) {
			if (line.equals("-1 -1 -1")) {
				break;
			}

			String[] token = line.split(" ");
			int m, f, r, sum;
			m = Integer.parseInt(token[0]);
			f = Integer.parseInt(token[1]);
			r = Integer.parseInt(token[2]);
			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");
			}
		}
	} String line; line BufferedReader br = new BufferedReader(new InputStreamReader(System.in)); br new BufferedReader(new InputStreamReader(System.in)) while ((line = br.readLine()) != null) {
			if (line.equals("-1 -1 -1")) {
				break;
			}

			String[] token = line.split(" ");
			int m, f, r, sum;
			m = Integer.parseInt(token[0]);
			f = Integer.parseInt(token[1]);
			r = Integer.parseInt(token[2]);
			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");
			}
		} (line = br.readLine()) != null (line = br.readLine()) line br.readLine() br.readLine br null {
			if (line.equals("-1 -1 -1")) {
				break;
			}

			String[] token = line.split(" ");
			int m, f, r, sum;
			m = Integer.parseInt(token[0]);
			f = Integer.parseInt(token[1]);
			r = Integer.parseInt(token[2]);
			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");
			}
		} if (line.equals("-1 -1 -1")) {
				break;
			} line.equals("-1 -1 -1") line.equals line "-1 -1 -1" {
				break;
			} break; String[] token = line.split(" "); token line.split(" ") line.split line " " int m m f f r r sum; sum m = Integer.parseInt(token[0]) m Integer.parseInt(token[0]) Integer.parseInt Integer token[0] token 0 f = Integer.parseInt(token[1]) f Integer.parseInt(token[1]) Integer.parseInt Integer token[1] token 1 r = Integer.parseInt(token[2]) r Integer.parseInt(token[2]) Integer.parseInt Integer token[2] token 2 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) throws IOException {
		// TODO Auto-generated method stub
		String line;
		BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
		while ((line = br.readLine()) != null) {
			if (line.equals("-1 -1 -1")) {
				break;
			}

			String[] token = line.split(" ");
			int m, f, r, sum;
			m = Integer.parseInt(token[0]);
			f = Integer.parseInt(token[1]);
			r = Integer.parseInt(token[2]);
			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");
			}
		}
	}
} public class Main {

	public static void main(String[] args) throws IOException {
		// TODO Auto-generated method stub
		String line;
		BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
		while ((line = br.readLine()) != null) {
			if (line.equals("-1 -1 -1")) {
				break;
			}

			String[] token = line.split(" ");
			int m, f, r, sum;
			m = Integer.parseInt(token[0]);
			f = Integer.parseInt(token[1]);
			r = Integer.parseInt(token[2]);
			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");
			}
		}
	}
} Main
import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;

public class Main {

	public static void main(String[] args) throws IOException {
		// TODO Auto-generated method stub
		String line;
		BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
		while ((line = br.readLine()) != null) {
			if (line.equals("-1 -1 -1")) {
				break;
			}

			String[] token = line.split(" ");
			int m, f, r, sum;
			m = Integer.parseInt(token[0]);
			f = Integer.parseInt(token[1]);
			r = Integer.parseInt(token[2]);
			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");
			}
		}
	}
}