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

while (true) {
int m = sc.nextInt();
int f = sc.nextInt();
int r = sc.nextInt();

if (m == -1 && f == -1 && r == -1) {
    break;
} else if (m == -1 || f == -1){
    System.out.println("F");
} else if(m+f>=80){
    System.out.println("A");
} else if(65<=m+f && m+f<80){
    System.out.println("B");
} else if(50<=m+f && m+f<65){
    System.out.println("C");
} else if(30<=m+f && m+f<50 && r<50){
    System.out.println("D");
} else if(30<=m+f && m+f<50 && r>=50){
    System.out.println("C");
} else {
    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);

while (true) {
int m = sc.nextInt();
int f = sc.nextInt();
int r = sc.nextInt();

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

while (true) {
int m = sc.nextInt();
int f = sc.nextInt();
int r = sc.nextInt();

if (m == -1 && f == -1 && r == -1) {
    break;
} else if (m == -1 || f == -1){
    System.out.println("F");
} else if(m+f>=80){
    System.out.println("A");
} else if(65<=m+f && m+f<80){
    System.out.println("B");
} else if(50<=m+f && m+f<65){
    System.out.println("C");
} else if(30<=m+f && m+f<50 && r<50){
    System.out.println("D");
} else if(30<=m+f && m+f<50 && r>=50){
    System.out.println("C");
} else {
    System.out.println("F");
}
    }
} main {
        Scanner sc = new Scanner(System.in);

while (true) {
int m = sc.nextInt();
int f = sc.nextInt();
int r = sc.nextInt();

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

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

if (m == -1 && f == -1 && r == -1) {
    break;
} else if (m == -1 || f == -1){
    System.out.println("F");
} else if(m+f>=80){
    System.out.println("A");
} else if(65<=m+f && m+f<80){
    System.out.println("B");
} else if(50<=m+f && m+f<65){
    System.out.println("C");
} else if(30<=m+f && m+f<50 && r<50){
    System.out.println("D");
} else if(30<=m+f && m+f<50 && r>=50){
    System.out.println("C");
} else {
    System.out.println("F");
}
    } int m = sc.nextInt(); m sc.nextInt() sc.nextInt sc int f = sc.nextInt(); f sc.nextInt() sc.nextInt sc int r = sc.nextInt(); r sc.nextInt() sc.nextInt sc if (m == -1 && f == -1 && r == -1) {
    break;
} else if (m == -1 || f == -1){
    System.out.println("F");
} else if(m+f>=80){
    System.out.println("A");
} else if(65<=m+f && m+f<80){
    System.out.println("B");
} else if(50<=m+f && m+f<65){
    System.out.println("C");
} else if(30<=m+f && m+f<50 && r<50){
    System.out.println("D");
} else if(30<=m+f && m+f<50 && r>=50){
    System.out.println("C");
} else {
    System.out.println("F");
} 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(65<=m+f && m+f<80){
    System.out.println("B");
} else if(50<=m+f && m+f<65){
    System.out.println("C");
} else if(30<=m+f && m+f<50 && r<50){
    System.out.println("D");
} else if(30<=m+f && m+f<50 && r>=50){
    System.out.println("C");
} 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(65<=m+f && m+f<80){
    System.out.println("B");
} else if(50<=m+f && m+f<65){
    System.out.println("C");
} else if(30<=m+f && m+f<50 && r<50){
    System.out.println("D");
} else if(30<=m+f && m+f<50 && r>=50){
    System.out.println("C");
} 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(65<=m+f && m+f<80){
    System.out.println("B");
} else if(50<=m+f && m+f<65){
    System.out.println("C");
} else if(30<=m+f && m+f<50 && r<50){
    System.out.println("D");
} else if(30<=m+f && m+f<50 && r>=50){
    System.out.println("C");
} else {
    System.out.println("F");
} 65<=m+f && m+f<80 65<=m+f 65 m+f m f 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(50<=m+f && m+f<65){
    System.out.println("C");
} else if(30<=m+f && m+f<50 && r<50){
    System.out.println("D");
} else if(30<=m+f && m+f<50 && r>=50){
    System.out.println("C");
} else {
    System.out.println("F");
} 50<=m+f && m+f<65 50<=m+f 50 m+f m f m+f<65 m+f m f 65 {
    System.out.println("C");
} System.out.println("C") System.out.println System.out System System.out "C" if(30<=m+f && m+f<50 && r<50){
    System.out.println("D");
} else if(30<=m+f && m+f<50 && r>=50){
    System.out.println("C");
} else {
    System.out.println("F");
} 30<=m+f && m+f<50 && r<50 30<=m+f && m+f<50 && r<50 30<=m+f 30 m+f m f m+f<50 m+f m f 50 r<50 r 50 {
    System.out.println("D");
} System.out.println("D") System.out.println System.out System System.out "D" if(30<=m+f && m+f<50 && r>=50){
    System.out.println("C");
} else {
    System.out.println("F");
} 30<=m+f && m+f<50 && r>=50 30<=m+f && m+f<50 && r>=50 30<=m+f 30 m+f m f m+f<50 m+f m f 50 r>=50 r 50 {
    System.out.println("C");
} System.out.println("C") System.out.println System.out System System.out "C" {
    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);

while (true) {
int m = sc.nextInt();
int f = sc.nextInt();
int r = sc.nextInt();

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

while (true) {
int m = sc.nextInt();
int f = sc.nextInt();
int r = sc.nextInt();

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

while (true) {
int m = sc.nextInt();
int f = sc.nextInt();
int r = sc.nextInt();

if (m == -1 && f == -1 && r == -1) {
    break;
} else if (m == -1 || f == -1){
    System.out.println("F");
} else if(m+f>=80){
    System.out.println("A");
} else if(65<=m+f && m+f<80){
    System.out.println("B");
} else if(50<=m+f && m+f<65){
    System.out.println("C");
} else if(30<=m+f && m+f<50 && r<50){
    System.out.println("D");
} else if(30<=m+f && m+f<50 && r>=50){
    System.out.println("C");
} else {
    System.out.println("F");
}
    }
}
}
