public class Main{
  public void run(java.io.InputStream in, java.io.PrintStream out){
    java.util.Scanner sc = new java.util.Scanner(in);
/*answer*/
    int m, f, r;

    for(;;){
      m = sc.nextInt();
      f = sc.nextInt();
      r = sc.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");
      }
    }


    sc.close();
  }
  public static void main(String[] args){
    (new Main()).run(System.in, System.out);
  }
} public class Main{
  public void run(java.io.InputStream in, java.io.PrintStream out){
    java.util.Scanner sc = new java.util.Scanner(in);
/*answer*/
    int m, f, r;

    for(;;){
      m = sc.nextInt();
      f = sc.nextInt();
      r = sc.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");
      }
    }


    sc.close();
  }
  public static void main(String[] args){
    (new Main()).run(System.in, System.out);
  }
} Main public void run(java.io.InputStream in, java.io.PrintStream out){
    java.util.Scanner sc = new java.util.Scanner(in);
/*answer*/
    int m, f, r;

    for(;;){
      m = sc.nextInt();
      f = sc.nextInt();
      r = sc.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");
      }
    }


    sc.close();
  } run {
    java.util.Scanner sc = new java.util.Scanner(in);
/*answer*/
    int m, f, r;

    for(;;){
      m = sc.nextInt();
      f = sc.nextInt();
      r = sc.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");
      }
    }


    sc.close();
  } java.util.Scanner sc = new java.util.Scanner(in); sc new java.util.Scanner(in) int m m f f r; r for(;;){
      m = sc.nextInt();
      f = sc.nextInt();
      r = sc.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");
      }
    } ; {
      m = sc.nextInt();
      f = sc.nextInt();
      r = sc.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");
      }
    } {
      m = sc.nextInt();
      f = sc.nextInt();
      r = sc.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");
      }
    } 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 + 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" sc.close() sc.close sc java.io.InputStream in in java.io.PrintStream out out public static void main(String[] args){
    (new Main()).run(System.in, System.out);
  } main {
    (new Main()).run(System.in, System.out);
  } (new Main()).run(System.in, System.out) (new Main()).run (new Main()) System.in System System.in System.out System System.out String[] args args public class Main{
  public void run(java.io.InputStream in, java.io.PrintStream out){
    java.util.Scanner sc = new java.util.Scanner(in);
/*answer*/
    int m, f, r;

    for(;;){
      m = sc.nextInt();
      f = sc.nextInt();
      r = sc.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");
      }
    }


    sc.close();
  }
  public static void main(String[] args){
    (new Main()).run(System.in, System.out);
  }
} public class Main{
  public void run(java.io.InputStream in, java.io.PrintStream out){
    java.util.Scanner sc = new java.util.Scanner(in);
/*answer*/
    int m, f, r;

    for(;;){
      m = sc.nextInt();
      f = sc.nextInt();
      r = sc.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");
      }
    }


    sc.close();
  }
  public static void main(String[] args){
    (new Main()).run(System.in, System.out);
  }
} Main
public class Main{
  public void run(java.io.InputStream in, java.io.PrintStream out){
    java.util.Scanner sc = new java.util.Scanner(in);
/*answer*/
    int m, f, r;

    for(;;){
      m = sc.nextInt();
      f = sc.nextInt();
      r = sc.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");
      }
    }


    sc.close();
  }
  public static void main(String[] args){
    (new Main()).run(System.in, System.out);
  }
}