import java.util.Scanner;

public class Main{
  public static void main(String[] args){
      Scanner sc     = new Scanner(System.in);
  for(int i = 0;i<50;i++){
      int m          = sc.nextInt();
      int f          = sc.nextInt();
      int r          = sc.nextInt();

      if(m==-1 && f==-1 && r==-1)
          break;

      if(m==-1||f==-1){
                System.out.println("F");
          }else if(80 <= m + f){
                System.out.println("A");
          }else if(65 <= m + f){
                System.out.println("B");
          }else if(50 <= m + f){
                System.out.println("C");
          }else if(30 <= m + f){
                if(50 <= r){
                  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 sc     = new Scanner(System.in);
  for(int i = 0;i<50;i++){
      int m          = sc.nextInt();
      int f          = sc.nextInt();
      int r          = sc.nextInt();

      if(m==-1 && f==-1 && r==-1)
          break;

      if(m==-1||f==-1){
                System.out.println("F");
          }else if(80 <= m + f){
                System.out.println("A");
          }else if(65 <= m + f){
                System.out.println("B");
          }else if(50 <= m + f){
                System.out.println("C");
          }else if(30 <= m + f){
                if(50 <= r){
                  System.out.println("C");
                }else{
                  System.out.println("D");
                }
          }else{
                System.out.println("F");
          }
  }
  }
} Main public static void main(String[] args){
      Scanner sc     = new Scanner(System.in);
  for(int i = 0;i<50;i++){
      int m          = sc.nextInt();
      int f          = sc.nextInt();
      int r          = sc.nextInt();

      if(m==-1 && f==-1 && r==-1)
          break;

      if(m==-1||f==-1){
                System.out.println("F");
          }else if(80 <= m + f){
                System.out.println("A");
          }else if(65 <= m + f){
                System.out.println("B");
          }else if(50 <= m + f){
                System.out.println("C");
          }else if(30 <= m + f){
                if(50 <= r){
                  System.out.println("C");
                }else{
                  System.out.println("D");
                }
          }else{
                System.out.println("F");
          }
  }
  } main {
      Scanner sc     = new Scanner(System.in);
  for(int i = 0;i<50;i++){
      int m          = sc.nextInt();
      int f          = sc.nextInt();
      int r          = sc.nextInt();

      if(m==-1 && f==-1 && r==-1)
          break;

      if(m==-1||f==-1){
                System.out.println("F");
          }else if(80 <= m + f){
                System.out.println("A");
          }else if(65 <= m + f){
                System.out.println("B");
          }else if(50 <= m + f){
                System.out.println("C");
          }else if(30 <= m + f){
                if(50 <= r){
                  System.out.println("C");
                }else{
                  System.out.println("D");
                }
          }else{
                System.out.println("F");
          }
  }
  } Scanner sc     = new Scanner(System.in); sc new Scanner(System.in) for(int i = 0;i<50;i++){
      int m          = sc.nextInt();
      int f          = sc.nextInt();
      int r          = sc.nextInt();

      if(m==-1 && f==-1 && r==-1)
          break;

      if(m==-1||f==-1){
                System.out.println("F");
          }else if(80 <= m + f){
                System.out.println("A");
          }else if(65 <= m + f){
                System.out.println("B");
          }else if(50 <= m + f){
                System.out.println("C");
          }else if(30 <= m + f){
                if(50 <= r){
                  System.out.println("C");
                }else{
                  System.out.println("D");
                }
          }else{
                System.out.println("F");
          }
  } int i = 0; int i = 0; i 0 i<50 i 50 i++ i++ i {
      int m          = sc.nextInt();
      int f          = sc.nextInt();
      int r          = sc.nextInt();

      if(m==-1 && f==-1 && r==-1)
          break;

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

      if(m==-1 && f==-1 && r==-1)
          break;

      if(m==-1||f==-1){
                System.out.println("F");
          }else if(80 <= m + f){
                System.out.println("A");
          }else if(65 <= m + f){
                System.out.println("B");
          }else if(50 <= m + f){
                System.out.println("C");
          }else if(30 <= m + f){
                if(50 <= r){
                  System.out.println("C");
                }else{
                  System.out.println("D");
                }
          }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; 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(80 <= m + f){
                System.out.println("A");
          }else if(65 <= m + f){
                System.out.println("B");
          }else if(50 <= m + f){
                System.out.println("C");
          }else if(30 <= m + f){
                if(50 <= r){
                  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(80 <= m + f){
                System.out.println("A");
          }else if(65 <= m + f){
                System.out.println("B");
          }else if(50 <= m + f){
                System.out.println("C");
          }else if(30 <= m + f){
                if(50 <= r){
                  System.out.println("C");
                }else{
                  System.out.println("D");
                }
          }else{
                System.out.println("F");
          } 80 <= m + f 80 m + f m f {
                System.out.println("A");
          } System.out.println("A") System.out.println System.out System System.out "A" if(65 <= m + f){
                System.out.println("B");
          }else if(50 <= m + f){
                System.out.println("C");
          }else if(30 <= m + f){
                if(50 <= r){
                  System.out.println("C");
                }else{
                  System.out.println("D");
                }
          }else{
                System.out.println("F");
          } 65 <= m + f 65 m + f m f {
                System.out.println("B");
          } System.out.println("B") System.out.println System.out System System.out "B" if(50 <= m + f){
                System.out.println("C");
          }else if(30 <= m + f){
                if(50 <= r){
                  System.out.println("C");
                }else{
                  System.out.println("D");
                }
          }else{
                System.out.println("F");
          } 50 <= m + f 50 m + f m f {
                System.out.println("C");
          } System.out.println("C") System.out.println System.out System System.out "C" if(30 <= m + f){
                if(50 <= r){
                  System.out.println("C");
                }else{
                  System.out.println("D");
                }
          }else{
                System.out.println("F");
          } 30 <= m + f 30 m + f m f {
                if(50 <= r){
                  System.out.println("C");
                }else{
                  System.out.println("D");
                }
          } if(50 <= r){
                  System.out.println("C");
                }else{
                  System.out.println("D");
                } 50 <= r 50 r {
                  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 sc     = new Scanner(System.in);
  for(int i = 0;i<50;i++){
      int m          = sc.nextInt();
      int f          = sc.nextInt();
      int r          = sc.nextInt();

      if(m==-1 && f==-1 && r==-1)
          break;

      if(m==-1||f==-1){
                System.out.println("F");
          }else if(80 <= m + f){
                System.out.println("A");
          }else if(65 <= m + f){
                System.out.println("B");
          }else if(50 <= m + f){
                System.out.println("C");
          }else if(30 <= m + f){
                if(50 <= r){
                  System.out.println("C");
                }else{
                  System.out.println("D");
                }
          }else{
                System.out.println("F");
          }
  }
  }
} public class Main{
  public static void main(String[] args){
      Scanner sc     = new Scanner(System.in);
  for(int i = 0;i<50;i++){
      int m          = sc.nextInt();
      int f          = sc.nextInt();
      int r          = sc.nextInt();

      if(m==-1 && f==-1 && r==-1)
          break;

      if(m==-1||f==-1){
                System.out.println("F");
          }else if(80 <= m + f){
                System.out.println("A");
          }else if(65 <= m + f){
                System.out.println("B");
          }else if(50 <= m + f){
                System.out.println("C");
          }else if(30 <= m + f){
                if(50 <= r){
                  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 sc     = new Scanner(System.in);
  for(int i = 0;i<50;i++){
      int m          = sc.nextInt();
      int f          = sc.nextInt();
      int r          = sc.nextInt();

      if(m==-1 && f==-1 && r==-1)
          break;

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

