import java.util.Scanner;

public class Main{
	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		int m, f, r;
		int count = 0;
		int sum = 0;
		int[][] records = new int[50][3];
		for (int i = 0; true; i++) {
			m = sc.nextInt();
			f = sc.nextInt();
			r = sc.nextInt();
			if (m == -1 && f == -1 && r == -1) {
				break;
			}
			records[i][0] = m;
			records[i][1] = f;
			records[i][2] = r;
			count++;
		}
		for (int i = 0; i < count; i++) {
			if (records[i][0] == -1 || records[i][1] == -1) {
				System.out.println("F");
			} else {
				sum = records[i][0] + records[i][1];
				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 (records[i][2] >= 50) {
						System.out.println("C");
					} else {
						System.out.println("D");
					}
				} else {
					System.out.println("F");
				}
			}
		}
		sc.close();
	}
} import java.util.Scanner; Scanner java.util public class Main{
	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		int m, f, r;
		int count = 0;
		int sum = 0;
		int[][] records = new int[50][3];
		for (int i = 0; true; i++) {
			m = sc.nextInt();
			f = sc.nextInt();
			r = sc.nextInt();
			if (m == -1 && f == -1 && r == -1) {
				break;
			}
			records[i][0] = m;
			records[i][1] = f;
			records[i][2] = r;
			count++;
		}
		for (int i = 0; i < count; i++) {
			if (records[i][0] == -1 || records[i][1] == -1) {
				System.out.println("F");
			} else {
				sum = records[i][0] + records[i][1];
				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 (records[i][2] >= 50) {
						System.out.println("C");
					} else {
						System.out.println("D");
					}
				} else {
					System.out.println("F");
				}
			}
		}
		sc.close();
	}
} Main public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		int m, f, r;
		int count = 0;
		int sum = 0;
		int[][] records = new int[50][3];
		for (int i = 0; true; i++) {
			m = sc.nextInt();
			f = sc.nextInt();
			r = sc.nextInt();
			if (m == -1 && f == -1 && r == -1) {
				break;
			}
			records[i][0] = m;
			records[i][1] = f;
			records[i][2] = r;
			count++;
		}
		for (int i = 0; i < count; i++) {
			if (records[i][0] == -1 || records[i][1] == -1) {
				System.out.println("F");
			} else {
				sum = records[i][0] + records[i][1];
				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 (records[i][2] >= 50) {
						System.out.println("C");
					} else {
						System.out.println("D");
					}
				} else {
					System.out.println("F");
				}
			}
		}
		sc.close();
	} main {
		Scanner sc = new Scanner(System.in);
		int m, f, r;
		int count = 0;
		int sum = 0;
		int[][] records = new int[50][3];
		for (int i = 0; true; i++) {
			m = sc.nextInt();
			f = sc.nextInt();
			r = sc.nextInt();
			if (m == -1 && f == -1 && r == -1) {
				break;
			}
			records[i][0] = m;
			records[i][1] = f;
			records[i][2] = r;
			count++;
		}
		for (int i = 0; i < count; i++) {
			if (records[i][0] == -1 || records[i][1] == -1) {
				System.out.println("F");
			} else {
				sum = records[i][0] + records[i][1];
				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 (records[i][2] >= 50) {
						System.out.println("C");
					} else {
						System.out.println("D");
					}
				} else {
					System.out.println("F");
				}
			}
		}
		sc.close();
	} Scanner sc = new Scanner(System.in); sc new Scanner(System.in) int m m f f r; r int count = 0; count 0 int sum = 0; sum 0 int[][] records = new int[50][3]; records new int[50][3] 50 3 for (int i = 0; true; i++) {
			m = sc.nextInt();
			f = sc.nextInt();
			r = sc.nextInt();
			if (m == -1 && f == -1 && r == -1) {
				break;
			}
			records[i][0] = m;
			records[i][1] = f;
			records[i][2] = r;
			count++;
		} int i = 0; int i = 0; i 0 true i++ i++ i {
			m = sc.nextInt();
			f = sc.nextInt();
			r = sc.nextInt();
			if (m == -1 && f == -1 && r == -1) {
				break;
			}
			records[i][0] = m;
			records[i][1] = f;
			records[i][2] = r;
			count++;
		} {
			m = sc.nextInt();
			f = sc.nextInt();
			r = sc.nextInt();
			if (m == -1 && f == -1 && r == -1) {
				break;
			}
			records[i][0] = m;
			records[i][1] = f;
			records[i][2] = r;
			count++;
		} m = sc.nextInt() m sc.nextInt() sc.nextInt sc f = sc.nextInt() f sc.nextInt() sc.nextInt sc r = sc.nextInt() r sc.nextInt() sc.nextInt sc if (m == -1 && f == -1 && r == -1) {
				break;
			} 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; records[i][0] = m records[i][0] records[i] records i 0 m records[i][1] = f records[i][1] records[i] records i 1 f records[i][2] = r records[i][2] records[i] records i 2 r count++ count for (int i = 0; i < count; i++) {
			if (records[i][0] == -1 || records[i][1] == -1) {
				System.out.println("F");
			} else {
				sum = records[i][0] + records[i][1];
				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 (records[i][2] >= 50) {
						System.out.println("C");
					} else {
						System.out.println("D");
					}
				} else {
					System.out.println("F");
				}
			}
		} int i = 0; int i = 0; i 0 i < count i count i++ i++ i {
			if (records[i][0] == -1 || records[i][1] == -1) {
				System.out.println("F");
			} else {
				sum = records[i][0] + records[i][1];
				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 (records[i][2] >= 50) {
						System.out.println("C");
					} else {
						System.out.println("D");
					}
				} else {
					System.out.println("F");
				}
			}
		} {
			if (records[i][0] == -1 || records[i][1] == -1) {
				System.out.println("F");
			} else {
				sum = records[i][0] + records[i][1];
				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 (records[i][2] >= 50) {
						System.out.println("C");
					} else {
						System.out.println("D");
					}
				} else {
					System.out.println("F");
				}
			}
		} if (records[i][0] == -1 || records[i][1] == -1) {
				System.out.println("F");
			} else {
				sum = records[i][0] + records[i][1];
				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 (records[i][2] >= 50) {
						System.out.println("C");
					} else {
						System.out.println("D");
					}
				} else {
					System.out.println("F");
				}
			} records[i][0] == -1 || records[i][1] == -1 records[i][0] == -1 records[i][0] records[i] records i 0 -1 1 records[i][1] == -1 records[i][1] records[i] records i 1 -1 1 {
				System.out.println("F");
			} System.out.println("F") System.out.println System.out System System.out "F" {
				sum = records[i][0] + records[i][1];
				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 (records[i][2] >= 50) {
						System.out.println("C");
					} else {
						System.out.println("D");
					}
				} else {
					System.out.println("F");
				}
			} sum = records[i][0] + records[i][1] sum records[i][0] + records[i][1] records[i][0] records[i] records i 0 records[i][1] records[i] records i 1 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 (records[i][2] >= 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 (records[i][2] >= 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 (records[i][2] >= 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 (records[i][2] >= 50) {
						System.out.println("C");
					} else {
						System.out.println("D");
					}
				} else {
					System.out.println("F");
				} sum >= 30 sum 30 {
					if (records[i][2] >= 50) {
						System.out.println("C");
					} else {
						System.out.println("D");
					}
				} if (records[i][2] >= 50) {
						System.out.println("C");
					} else {
						System.out.println("D");
					} records[i][2] >= 50 records[i][2] records[i] records i 2 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" sc.close() sc.close sc String[] args args public class Main{
	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		int m, f, r;
		int count = 0;
		int sum = 0;
		int[][] records = new int[50][3];
		for (int i = 0; true; i++) {
			m = sc.nextInt();
			f = sc.nextInt();
			r = sc.nextInt();
			if (m == -1 && f == -1 && r == -1) {
				break;
			}
			records[i][0] = m;
			records[i][1] = f;
			records[i][2] = r;
			count++;
		}
		for (int i = 0; i < count; i++) {
			if (records[i][0] == -1 || records[i][1] == -1) {
				System.out.println("F");
			} else {
				sum = records[i][0] + records[i][1];
				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 (records[i][2] >= 50) {
						System.out.println("C");
					} else {
						System.out.println("D");
					}
				} else {
					System.out.println("F");
				}
			}
		}
		sc.close();
	}
} public class Main{
	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		int m, f, r;
		int count = 0;
		int sum = 0;
		int[][] records = new int[50][3];
		for (int i = 0; true; i++) {
			m = sc.nextInt();
			f = sc.nextInt();
			r = sc.nextInt();
			if (m == -1 && f == -1 && r == -1) {
				break;
			}
			records[i][0] = m;
			records[i][1] = f;
			records[i][2] = r;
			count++;
		}
		for (int i = 0; i < count; i++) {
			if (records[i][0] == -1 || records[i][1] == -1) {
				System.out.println("F");
			} else {
				sum = records[i][0] + records[i][1];
				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 (records[i][2] >= 50) {
						System.out.println("C");
					} else {
						System.out.println("D");
					}
				} else {
					System.out.println("F");
				}
			}
		}
		sc.close();
	}
} Main
import java.util.Scanner;

public class Main{
	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		int m, f, r;
		int count = 0;
		int sum = 0;
		int[][] records = new int[50][3];
		for (int i = 0; true; i++) {
			m = sc.nextInt();
			f = sc.nextInt();
			r = sc.nextInt();
			if (m == -1 && f == -1 && r == -1) {
				break;
			}
			records[i][0] = m;
			records[i][1] = f;
			records[i][2] = r;
			count++;
		}
		for (int i = 0; i < count; i++) {
			if (records[i][0] == -1 || records[i][1] == -1) {
				System.out.println("F");
			} else {
				sum = records[i][0] + records[i][1];
				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 (records[i][2] >= 50) {
						System.out.println("C");
					} else {
						System.out.println("D");
					}
				} else {
					System.out.println("F");
				}
			}
		}
		sc.close();
	}
}
