
import java.util.Scanner;

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

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

} Main

import java.util.Scanner;

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

}