import java.util.Scanner;

public class Main
{
	public static void main ( String[] args ) {
		Scanner sc = new Scanner ( System.in );
		
		int m = 0;
		int f = 0;
		int r = 0;
		
		while ( true ) {
			m = sc.nextInt ();
			f = sc.nextInt ();
			r = sc.nextInt ();
			
			if ( m < 0 && f < 0 && r < 0 ) {
				break;
			}
			
			int sum = m + f;
			
			if ( m == -1 || f == -1 ) {
				System.out.println ( "F" );
			} else if ( 80 <= sum ) {
				System.out.println ( "A" );
			} else if ( 65 <= sum && sum < 80 ) {
				System.out.println ( "B" );
			} else if ( 50 <= sum && sum < 65 ) {
				System.out.println ( "C" );
			} else if ( 30 <= sum && sum < 50 ) {
				if ( 50 <= r ) {
					System.out.println ( "C" );
				} else {
					System.out.println ( "D" );
				}
			} else if ( 0 <= sum && sum < 30 ) {
				System.out.println ( "F" );
			}
		}
	}
} import java.util.Scanner; Scanner java.util public class Main
{
	public static void main ( String[] args ) {
		Scanner sc = new Scanner ( System.in );
		
		int m = 0;
		int f = 0;
		int r = 0;
		
		while ( true ) {
			m = sc.nextInt ();
			f = sc.nextInt ();
			r = sc.nextInt ();
			
			if ( m < 0 && f < 0 && r < 0 ) {
				break;
			}
			
			int sum = m + f;
			
			if ( m == -1 || f == -1 ) {
				System.out.println ( "F" );
			} else if ( 80 <= sum ) {
				System.out.println ( "A" );
			} else if ( 65 <= sum && sum < 80 ) {
				System.out.println ( "B" );
			} else if ( 50 <= sum && sum < 65 ) {
				System.out.println ( "C" );
			} else if ( 30 <= sum && sum < 50 ) {
				if ( 50 <= r ) {
					System.out.println ( "C" );
				} else {
					System.out.println ( "D" );
				}
			} else if ( 0 <= sum && sum < 30 ) {
				System.out.println ( "F" );
			}
		}
	}
} Main public static void main ( String[] args ) {
		Scanner sc = new Scanner ( System.in );
		
		int m = 0;
		int f = 0;
		int r = 0;
		
		while ( true ) {
			m = sc.nextInt ();
			f = sc.nextInt ();
			r = sc.nextInt ();
			
			if ( m < 0 && f < 0 && r < 0 ) {
				break;
			}
			
			int sum = m + f;
			
			if ( m == -1 || f == -1 ) {
				System.out.println ( "F" );
			} else if ( 80 <= sum ) {
				System.out.println ( "A" );
			} else if ( 65 <= sum && sum < 80 ) {
				System.out.println ( "B" );
			} else if ( 50 <= sum && sum < 65 ) {
				System.out.println ( "C" );
			} else if ( 30 <= sum && sum < 50 ) {
				if ( 50 <= r ) {
					System.out.println ( "C" );
				} else {
					System.out.println ( "D" );
				}
			} else if ( 0 <= sum && sum < 30 ) {
				System.out.println ( "F" );
			}
		}
	} main {
		Scanner sc = new Scanner ( System.in );
		
		int m = 0;
		int f = 0;
		int r = 0;
		
		while ( true ) {
			m = sc.nextInt ();
			f = sc.nextInt ();
			r = sc.nextInt ();
			
			if ( m < 0 && f < 0 && r < 0 ) {
				break;
			}
			
			int sum = m + f;
			
			if ( m == -1 || f == -1 ) {
				System.out.println ( "F" );
			} else if ( 80 <= sum ) {
				System.out.println ( "A" );
			} else if ( 65 <= sum && sum < 80 ) {
				System.out.println ( "B" );
			} else if ( 50 <= sum && sum < 65 ) {
				System.out.println ( "C" );
			} else if ( 30 <= sum && sum < 50 ) {
				if ( 50 <= r ) {
					System.out.println ( "C" );
				} else {
					System.out.println ( "D" );
				}
			} else if ( 0 <= sum && sum < 30 ) {
				System.out.println ( "F" );
			}
		}
	} Scanner sc = new Scanner ( System.in ); sc new Scanner ( System.in ) int m = 0; m 0 int f = 0; f 0 int r = 0; r 0 while ( true ) {
			m = sc.nextInt ();
			f = sc.nextInt ();
			r = sc.nextInt ();
			
			if ( m < 0 && f < 0 && r < 0 ) {
				break;
			}
			
			int sum = m + f;
			
			if ( m == -1 || f == -1 ) {
				System.out.println ( "F" );
			} else if ( 80 <= sum ) {
				System.out.println ( "A" );
			} else if ( 65 <= sum && sum < 80 ) {
				System.out.println ( "B" );
			} else if ( 50 <= sum && sum < 65 ) {
				System.out.println ( "C" );
			} else if ( 30 <= sum && sum < 50 ) {
				if ( 50 <= r ) {
					System.out.println ( "C" );
				} else {
					System.out.println ( "D" );
				}
			} else if ( 0 <= sum && sum < 30 ) {
				System.out.println ( "F" );
			}
		} true {
			m = sc.nextInt ();
			f = sc.nextInt ();
			r = sc.nextInt ();
			
			if ( m < 0 && f < 0 && r < 0 ) {
				break;
			}
			
			int sum = m + f;
			
			if ( m == -1 || f == -1 ) {
				System.out.println ( "F" );
			} else if ( 80 <= sum ) {
				System.out.println ( "A" );
			} else if ( 65 <= sum && sum < 80 ) {
				System.out.println ( "B" );
			} else if ( 50 <= sum && sum < 65 ) {
				System.out.println ( "C" );
			} else if ( 30 <= sum && sum < 50 ) {
				if ( 50 <= r ) {
					System.out.println ( "C" );
				} else {
					System.out.println ( "D" );
				}
			} else if ( 0 <= sum && sum < 30 ) {
				System.out.println ( "F" );
			}
		} 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 < 0 && f < 0 && r < 0 ) {
				break;
			} m < 0 && f < 0 && r < 0 m < 0 && f < 0 && r < 0 m < 0 m 0 f < 0 f 0 r < 0 r 0 {
				break;
			} break; int sum = m + f; sum m + f m f if ( m == -1 || f == -1 ) {
				System.out.println ( "F" );
			} else if ( 80 <= sum ) {
				System.out.println ( "A" );
			} else if ( 65 <= sum && sum < 80 ) {
				System.out.println ( "B" );
			} else if ( 50 <= sum && sum < 65 ) {
				System.out.println ( "C" );
			} else if ( 30 <= sum && sum < 50 ) {
				if ( 50 <= r ) {
					System.out.println ( "C" );
				} else {
					System.out.println ( "D" );
				}
			} else if ( 0 <= sum && sum < 30 ) {
				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 ( 80 <= sum ) {
				System.out.println ( "A" );
			} else if ( 65 <= sum && sum < 80 ) {
				System.out.println ( "B" );
			} else if ( 50 <= sum && sum < 65 ) {
				System.out.println ( "C" );
			} else if ( 30 <= sum && sum < 50 ) {
				if ( 50 <= r ) {
					System.out.println ( "C" );
				} else {
					System.out.println ( "D" );
				}
			} else if ( 0 <= sum && sum < 30 ) {
				System.out.println ( "F" );
			} 80 <= sum 80 sum {
				System.out.println ( "A" );
			} System.out.println ( "A" ) System.out.println System.out System System.out "A" if ( 65 <= sum && sum < 80 ) {
				System.out.println ( "B" );
			} else if ( 50 <= sum && sum < 65 ) {
				System.out.println ( "C" );
			} else if ( 30 <= sum && sum < 50 ) {
				if ( 50 <= r ) {
					System.out.println ( "C" );
				} else {
					System.out.println ( "D" );
				}
			} else if ( 0 <= sum && sum < 30 ) {
				System.out.println ( "F" );
			} 65 <= sum && sum < 80 65 <= sum 65 sum sum < 80 sum 80 {
				System.out.println ( "B" );
			} System.out.println ( "B" ) System.out.println System.out System System.out "B" if ( 50 <= sum && sum < 65 ) {
				System.out.println ( "C" );
			} else if ( 30 <= sum && sum < 50 ) {
				if ( 50 <= r ) {
					System.out.println ( "C" );
				} else {
					System.out.println ( "D" );
				}
			} else if ( 0 <= sum && sum < 30 ) {
				System.out.println ( "F" );
			} 50 <= sum && sum < 65 50 <= sum 50 sum sum < 65 sum 65 {
				System.out.println ( "C" );
			} System.out.println ( "C" ) System.out.println System.out System System.out "C" if ( 30 <= sum && sum < 50 ) {
				if ( 50 <= r ) {
					System.out.println ( "C" );
				} else {
					System.out.println ( "D" );
				}
			} else if ( 0 <= sum && sum < 30 ) {
				System.out.println ( "F" );
			} 30 <= sum && sum < 50 30 <= sum 30 sum sum < 50 sum 50 {
				if ( 50 <= r ) {
					System.out.println ( "C" );
				} else {
					System.out.println ( "D" );
				}
			} if ( 50 <= r ) {
					System.out.println ( "C" );
				} else {
					System.out.println ( "D" );
				} 50 <= r 50 r {
					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 ( 0 <= sum && sum < 30 ) {
				System.out.println ( "F" );
			} 0 <= sum && sum < 30 0 <= sum 0 sum sum < 30 sum 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 ) {
		Scanner sc = new Scanner ( System.in );
		
		int m = 0;
		int f = 0;
		int r = 0;
		
		while ( true ) {
			m = sc.nextInt ();
			f = sc.nextInt ();
			r = sc.nextInt ();
			
			if ( m < 0 && f < 0 && r < 0 ) {
				break;
			}
			
			int sum = m + f;
			
			if ( m == -1 || f == -1 ) {
				System.out.println ( "F" );
			} else if ( 80 <= sum ) {
				System.out.println ( "A" );
			} else if ( 65 <= sum && sum < 80 ) {
				System.out.println ( "B" );
			} else if ( 50 <= sum && sum < 65 ) {
				System.out.println ( "C" );
			} else if ( 30 <= sum && sum < 50 ) {
				if ( 50 <= r ) {
					System.out.println ( "C" );
				} else {
					System.out.println ( "D" );
				}
			} else if ( 0 <= sum && sum < 30 ) {
				System.out.println ( "F" );
			}
		}
	}
} public class Main
{
	public static void main ( String[] args ) {
		Scanner sc = new Scanner ( System.in );
		
		int m = 0;
		int f = 0;
		int r = 0;
		
		while ( true ) {
			m = sc.nextInt ();
			f = sc.nextInt ();
			r = sc.nextInt ();
			
			if ( m < 0 && f < 0 && r < 0 ) {
				break;
			}
			
			int sum = m + f;
			
			if ( m == -1 || f == -1 ) {
				System.out.println ( "F" );
			} else if ( 80 <= sum ) {
				System.out.println ( "A" );
			} else if ( 65 <= sum && sum < 80 ) {
				System.out.println ( "B" );
			} else if ( 50 <= sum && sum < 65 ) {
				System.out.println ( "C" );
			} else if ( 30 <= sum && sum < 50 ) {
				if ( 50 <= r ) {
					System.out.println ( "C" );
				} else {
					System.out.println ( "D" );
				}
			} else if ( 0 <= sum && sum < 30 ) {
				System.out.println ( "F" );
			}
		}
	}
} Main
import java.util.Scanner;

public class Main
{
	public static void main ( String[] args ) {
		Scanner sc = new Scanner ( System.in );
		
		int m = 0;
		int f = 0;
		int r = 0;
		
		while ( true ) {
			m = sc.nextInt ();
			f = sc.nextInt ();
			r = sc.nextInt ();
			
			if ( m < 0 && f < 0 && r < 0 ) {
				break;
			}
			
			int sum = m + f;
			
			if ( m == -1 || f == -1 ) {
				System.out.println ( "F" );
			} else if ( 80 <= sum ) {
				System.out.println ( "A" );
			} else if ( 65 <= sum && sum < 80 ) {
				System.out.println ( "B" );
			} else if ( 50 <= sum && sum < 65 ) {
				System.out.println ( "C" );
			} else if ( 30 <= sum && sum < 50 ) {
				if ( 50 <= r ) {
					System.out.println ( "C" );
				} else {
					System.out.println ( "D" );
				}
			} else if ( 0 <= sum && sum < 30 ) {
				System.out.println ( "F" );
			}
		}
	}
}