
import java.util.*;

public class Main {
	
	public static void main(String[] args) {
		Scanner cin=new Scanner(System.in);
		
		while(true) {
			int m=cin.nextInt(),f=cin.nextInt(),r=cin.nextInt();
			if(m+f+r==-3)break;
			int 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.util.*; util.* java public class Main {
	
	public static void main(String[] args) {
		Scanner cin=new Scanner(System.in);
		
		while(true) {
			int m=cin.nextInt(),f=cin.nextInt(),r=cin.nextInt();
			if(m+f+r==-3)break;
			int 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) {
		Scanner cin=new Scanner(System.in);
		
		while(true) {
			int m=cin.nextInt(),f=cin.nextInt(),r=cin.nextInt();
			if(m+f+r==-3)break;
			int 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 {
		Scanner cin=new Scanner(System.in);
		
		while(true) {
			int m=cin.nextInt(),f=cin.nextInt(),r=cin.nextInt();
			if(m+f+r==-3)break;
			int 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");
		}
	} Scanner cin=new Scanner(System.in); cin new Scanner(System.in) while(true) {
			int m=cin.nextInt(),f=cin.nextInt(),r=cin.nextInt();
			if(m+f+r==-3)break;
			int 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");
		} true {
			int m=cin.nextInt(),f=cin.nextInt(),r=cin.nextInt();
			if(m+f+r==-3)break;
			int 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");
		} int m=cin.nextInt() m cin.nextInt() cin.nextInt cin f=cin.nextInt() f cin.nextInt() cin.nextInt cin r=cin.nextInt(); r cin.nextInt() cin.nextInt cin if(m+f+r==-3)break; m+f+r==-3 m+f+r m+f+r m f r -3 3 break; int 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 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 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 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 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 System.out System System.out "C" System.out.println("D") System.out.println System.out System System.out "D" 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 cin=new Scanner(System.in);
		
		while(true) {
			int m=cin.nextInt(),f=cin.nextInt(),r=cin.nextInt();
			if(m+f+r==-3)break;
			int 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) {
		Scanner cin=new Scanner(System.in);
		
		while(true) {
			int m=cin.nextInt(),f=cin.nextInt(),r=cin.nextInt();
			if(m+f+r==-3)break;
			int 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.util.*;

public class Main {
	
	public static void main(String[] args) {
		Scanner cin=new Scanner(System.in);
		
		while(true) {
			int m=cin.nextInt(),f=cin.nextInt(),r=cin.nextInt();
			if(m+f+r==-3)break;
			int 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");
		}
	}
}
