import java.util.*;

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+f+r) == -3){
				break;
			}else 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.*; util.* java 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+f+r) == -3){
				break;
			}else 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+f+r) == -3){
				break;
			}else 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+f+r) == -3){
				break;
			}else 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+f+r) == -3){
				break;
			}else 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+f+r) == -3){
				break;
			}else 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+f+r) == -3){
				break;
			}else 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+f+r) == -3 (m+f+r) m+f+r m f r -3 3 {
				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
import java.util.*;

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+f+r) == -3){
				break;
			}else 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");
			}

		}
	}
}