import java.util.Scanner;

public class Main{
	public static void main(String[]args) {
		int m, f, r;
		Scanner sc = new Scanner(System.in);

		while(true) {
		m = sc.nextInt();
		f = sc.nextInt();
		r = sc.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 && (m+f)<80) {
			System.out.println('B');
		}else if((m + f)>=50 && (m+f)<65 ||r>=50) {
			System.out.println('C');
		}else if((m+f)>=30 && (m+f)<50) {
			System.out.println('D');
		}else if((m+f)<30) {
			System.out.println('F');
		}
		}

	}

} import java.util.Scanner; Scanner java.util public class Main{
	public static void main(String[]args) {
		int m, f, r;
		Scanner sc = new Scanner(System.in);

		while(true) {
		m = sc.nextInt();
		f = sc.nextInt();
		r = sc.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 && (m+f)<80) {
			System.out.println('B');
		}else if((m + f)>=50 && (m+f)<65 ||r>=50) {
			System.out.println('C');
		}else if((m+f)>=30 && (m+f)<50) {
			System.out.println('D');
		}else if((m+f)<30) {
			System.out.println('F');
		}
		}

	}

} Main public static void main(String[]args) {
		int m, f, r;
		Scanner sc = new Scanner(System.in);

		while(true) {
		m = sc.nextInt();
		f = sc.nextInt();
		r = sc.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 && (m+f)<80) {
			System.out.println('B');
		}else if((m + f)>=50 && (m+f)<65 ||r>=50) {
			System.out.println('C');
		}else if((m+f)>=30 && (m+f)<50) {
			System.out.println('D');
		}else if((m+f)<30) {
			System.out.println('F');
		}
		}

	} main {
		int m, f, r;
		Scanner sc = new Scanner(System.in);

		while(true) {
		m = sc.nextInt();
		f = sc.nextInt();
		r = sc.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 && (m+f)<80) {
			System.out.println('B');
		}else if((m + f)>=50 && (m+f)<65 ||r>=50) {
			System.out.println('C');
		}else if((m+f)>=30 && (m+f)<50) {
			System.out.println('D');
		}else if((m+f)<30) {
			System.out.println('F');
		}
		}

	} int m m f f r; r Scanner sc = new Scanner(System.in); sc new Scanner(System.in) while(true) {
		m = sc.nextInt();
		f = sc.nextInt();
		r = sc.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 && (m+f)<80) {
			System.out.println('B');
		}else if((m + f)>=50 && (m+f)<65 ||r>=50) {
			System.out.println('C');
		}else if((m+f)>=30 && (m+f)<50) {
			System.out.println('D');
		}else if((m+f)<30) {
			System.out.println('F');
		}
		} true {
		m = sc.nextInt();
		f = sc.nextInt();
		r = sc.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 && (m+f)<80) {
			System.out.println('B');
		}else if((m + f)>=50 && (m+f)<65 ||r>=50) {
			System.out.println('C');
		}else if((m+f)>=30 && (m+f)<50) {
			System.out.println('D');
		}else if((m+f)<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==-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; if(m==-1 || f==-1) {
			System.out.println('F');
		}else if((m + f)>=80) {
			System.out.println('A');
		}else if((m + f)>=65 && (m+f)<80) {
			System.out.println('B');
		}else if((m + f)>=50 && (m+f)<65 ||r>=50) {
			System.out.println('C');
		}else if((m+f)>=30 && (m+f)<50) {
			System.out.println('D');
		}else if((m+f)<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((m + f)>=80) {
			System.out.println('A');
		}else if((m + f)>=65 && (m+f)<80) {
			System.out.println('B');
		}else if((m + f)>=50 && (m+f)<65 ||r>=50) {
			System.out.println('C');
		}else if((m+f)>=30 && (m+f)<50) {
			System.out.println('D');
		}else if((m+f)<30) {
			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 && (m+f)<80) {
			System.out.println('B');
		}else if((m + f)>=50 && (m+f)<65 ||r>=50) {
			System.out.println('C');
		}else if((m+f)>=30 && (m+f)<50) {
			System.out.println('D');
		}else if((m+f)<30) {
			System.out.println('F');
		} (m + f)>=65 && (m+f)<80 (m + f)>=65 (m + f) m f 65 (m+f)<80 (m+f) m f 80 {
			System.out.println('B');
		} System.out.println('B') System.out.println System.out System System.out 'B' if((m + f)>=50 && (m+f)<65 ||r>=50) {
			System.out.println('C');
		}else if((m+f)>=30 && (m+f)<50) {
			System.out.println('D');
		}else if((m+f)<30) {
			System.out.println('F');
		} (m + f)>=50 && (m+f)<65 ||r>=50 (m + f)>=50 && (m+f)<65 (m + f)>=50 (m + f) m f 50 (m+f)<65 (m+f) m f 65 r>=50 r 50 {
			System.out.println('C');
		} System.out.println('C') System.out.println System.out System System.out 'C' if((m+f)>=30 && (m+f)<50) {
			System.out.println('D');
		}else if((m+f)<30) {
			System.out.println('F');
		} (m+f)>=30 && (m+f)<50 (m+f)>=30 (m+f) m f 30 (m+f)<50 (m+f) m f 50 {
			System.out.println('D');
		} System.out.println('D') System.out.println System.out System System.out 'D' if((m+f)<30) {
			System.out.println('F');
		} (m+f)<30 (m+f) m f 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) {
		int m, f, r;
		Scanner sc = new Scanner(System.in);

		while(true) {
		m = sc.nextInt();
		f = sc.nextInt();
		r = sc.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 && (m+f)<80) {
			System.out.println('B');
		}else if((m + f)>=50 && (m+f)<65 ||r>=50) {
			System.out.println('C');
		}else if((m+f)>=30 && (m+f)<50) {
			System.out.println('D');
		}else if((m+f)<30) {
			System.out.println('F');
		}
		}

	}

} public class Main{
	public static void main(String[]args) {
		int m, f, r;
		Scanner sc = new Scanner(System.in);

		while(true) {
		m = sc.nextInt();
		f = sc.nextInt();
		r = sc.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 && (m+f)<80) {
			System.out.println('B');
		}else if((m + f)>=50 && (m+f)<65 ||r>=50) {
			System.out.println('C');
		}else if((m+f)>=30 && (m+f)<50) {
			System.out.println('D');
		}else if((m+f)<30) {
			System.out.println('F');
		}
		}

	}

} Main
import java.util.Scanner;

public class Main{
	public static void main(String[]args) {
		int m, f, r;
		Scanner sc = new Scanner(System.in);

		while(true) {
		m = sc.nextInt();
		f = sc.nextInt();
		r = sc.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 && (m+f)<80) {
			System.out.println('B');
		}else if((m + f)>=50 && (m+f)<65 ||r>=50) {
			System.out.println('C');
		}else if((m+f)>=30 && (m+f)<50) {
			System.out.println('D');
		}else if((m+f)<30) {
			System.out.println('F');
		}
		}

	}

}
