import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStream;
import java.io.InputStreamReader;
import java.io.PrintWriter;
import java.util.StringTokenizer;

public class Main {
  static InputReader in;
  static PrintWriter out;

  static class Solution {
    void solve() throws IOException {
      while (true) {
        int m = in.nextInt();
        int f = in.nextInt();
        int r = in.nextInt();
        char grade = 'F';
        int tmp = m + f;
        
        if (m == -1 || f == -1 ) {
          if (m == -1 && f == -1 &&  r == -1) {
            break;
          } 
        } else if (tmp >= 80) {
          grade = 'A';
        } else if (tmp >= 65) {
          grade = 'B';
        } else if (tmp >= 50) {
          grade = 'C';
        } else if (tmp >= 30) {
          grade = 'D';
          
          if (r >= 50) {
            grade = 'C';
          } 
        } 
        
       
        
        out.println(grade);
      }
      
//      End solve()
    }
//    End Solution
}

  public static void main(String[] args) throws IOException {
    in = new InputReader(System.in);
    out = new PrintWriter(System.out);
    Solution solver = new Solution();
    solver.solve();
    out.close();
}

  static class InputReader {
    public BufferedReader br;
    public StringTokenizer st;

    public InputReader(InputStream stream) {
      br = new BufferedReader(new InputStreamReader(stream));
    }

    public int nextInt() throws IOException {
      while (st == null || !st.hasMoreTokens()) {
        st = new StringTokenizer(br.readLine());
      }
      return Integer.parseInt(st.nextToken());
    }

    public long nextLong() throws IOException {
      while (st == null || !st.hasMoreTokens()) {
        st = new StringTokenizer(br.readLine());
      }
      return Long.parseLong(st.nextToken());
    }

    public String next() throws IOException {
      while (st == null || !st.hasMoreTokens()) {
        st = new StringTokenizer(br.readLine());
      }
      return st.nextToken();
    }

    public double nextDouble() throws IOException {
      while (st == null || !st.hasMoreTokens()) {
        st = new StringTokenizer(br.readLine());
      }
      return Double.parseDouble(st.nextToken());
    }


  }

} import java.io.BufferedReader; BufferedReader java.io import java.io.IOException; IOException java.io import java.io.InputStream; InputStream java.io import java.io.InputStreamReader; InputStreamReader java.io import java.io.PrintWriter; PrintWriter java.io import java.util.StringTokenizer; StringTokenizer java.util public class Main {
  static InputReader in;
  static PrintWriter out;

  static class Solution {
    void solve() throws IOException {
      while (true) {
        int m = in.nextInt();
        int f = in.nextInt();
        int r = in.nextInt();
        char grade = 'F';
        int tmp = m + f;
        
        if (m == -1 || f == -1 ) {
          if (m == -1 && f == -1 &&  r == -1) {
            break;
          } 
        } else if (tmp >= 80) {
          grade = 'A';
        } else if (tmp >= 65) {
          grade = 'B';
        } else if (tmp >= 50) {
          grade = 'C';
        } else if (tmp >= 30) {
          grade = 'D';
          
          if (r >= 50) {
            grade = 'C';
          } 
        } 
        
       
        
        out.println(grade);
      }
      
//      End solve()
    }
//    End Solution
}

  public static void main(String[] args) throws IOException {
    in = new InputReader(System.in);
    out = new PrintWriter(System.out);
    Solution solver = new Solution();
    solver.solve();
    out.close();
}

  static class InputReader {
    public BufferedReader br;
    public StringTokenizer st;

    public InputReader(InputStream stream) {
      br = new BufferedReader(new InputStreamReader(stream));
    }

    public int nextInt() throws IOException {
      while (st == null || !st.hasMoreTokens()) {
        st = new StringTokenizer(br.readLine());
      }
      return Integer.parseInt(st.nextToken());
    }

    public long nextLong() throws IOException {
      while (st == null || !st.hasMoreTokens()) {
        st = new StringTokenizer(br.readLine());
      }
      return Long.parseLong(st.nextToken());
    }

    public String next() throws IOException {
      while (st == null || !st.hasMoreTokens()) {
        st = new StringTokenizer(br.readLine());
      }
      return st.nextToken();
    }

    public double nextDouble() throws IOException {
      while (st == null || !st.hasMoreTokens()) {
        st = new StringTokenizer(br.readLine());
      }
      return Double.parseDouble(st.nextToken());
    }


  }

} Main static InputReader in; in static PrintWriter out; out public static void main(String[] args) throws IOException {
    in = new InputReader(System.in);
    out = new PrintWriter(System.out);
    Solution solver = new Solution();
    solver.solve();
    out.close();
} main {
    in = new InputReader(System.in);
    out = new PrintWriter(System.out);
    Solution solver = new Solution();
    solver.solve();
    out.close();
} in = new InputReader(System.in) in new InputReader(System.in) out = new PrintWriter(System.out) out new PrintWriter(System.out) Solution solver = new Solution(); solver new Solution() solver.solve() solver.solve solver out.close() out.close out String[] args args static class Solution {
    void solve() throws IOException {
      while (true) {
        int m = in.nextInt();
        int f = in.nextInt();
        int r = in.nextInt();
        char grade = 'F';
        int tmp = m + f;
        
        if (m == -1 || f == -1 ) {
          if (m == -1 && f == -1 &&  r == -1) {
            break;
          } 
        } else if (tmp >= 80) {
          grade = 'A';
        } else if (tmp >= 65) {
          grade = 'B';
        } else if (tmp >= 50) {
          grade = 'C';
        } else if (tmp >= 30) {
          grade = 'D';
          
          if (r >= 50) {
            grade = 'C';
          } 
        } 
        
       
        
        out.println(grade);
      }
      
//      End solve()
    }
//    End Solution
} Solution void solve() throws IOException {
      while (true) {
        int m = in.nextInt();
        int f = in.nextInt();
        int r = in.nextInt();
        char grade = 'F';
        int tmp = m + f;
        
        if (m == -1 || f == -1 ) {
          if (m == -1 && f == -1 &&  r == -1) {
            break;
          } 
        } else if (tmp >= 80) {
          grade = 'A';
        } else if (tmp >= 65) {
          grade = 'B';
        } else if (tmp >= 50) {
          grade = 'C';
        } else if (tmp >= 30) {
          grade = 'D';
          
          if (r >= 50) {
            grade = 'C';
          } 
        } 
        
       
        
        out.println(grade);
      }
      
//      End solve()
    } solve {
      while (true) {
        int m = in.nextInt();
        int f = in.nextInt();
        int r = in.nextInt();
        char grade = 'F';
        int tmp = m + f;
        
        if (m == -1 || f == -1 ) {
          if (m == -1 && f == -1 &&  r == -1) {
            break;
          } 
        } else if (tmp >= 80) {
          grade = 'A';
        } else if (tmp >= 65) {
          grade = 'B';
        } else if (tmp >= 50) {
          grade = 'C';
        } else if (tmp >= 30) {
          grade = 'D';
          
          if (r >= 50) {
            grade = 'C';
          } 
        } 
        
       
        
        out.println(grade);
      }
      
//      End solve()
    } while (true) {
        int m = in.nextInt();
        int f = in.nextInt();
        int r = in.nextInt();
        char grade = 'F';
        int tmp = m + f;
        
        if (m == -1 || f == -1 ) {
          if (m == -1 && f == -1 &&  r == -1) {
            break;
          } 
        } else if (tmp >= 80) {
          grade = 'A';
        } else if (tmp >= 65) {
          grade = 'B';
        } else if (tmp >= 50) {
          grade = 'C';
        } else if (tmp >= 30) {
          grade = 'D';
          
          if (r >= 50) {
            grade = 'C';
          } 
        } 
        
       
        
        out.println(grade);
      } true {
        int m = in.nextInt();
        int f = in.nextInt();
        int r = in.nextInt();
        char grade = 'F';
        int tmp = m + f;
        
        if (m == -1 || f == -1 ) {
          if (m == -1 && f == -1 &&  r == -1) {
            break;
          } 
        } else if (tmp >= 80) {
          grade = 'A';
        } else if (tmp >= 65) {
          grade = 'B';
        } else if (tmp >= 50) {
          grade = 'C';
        } else if (tmp >= 30) {
          grade = 'D';
          
          if (r >= 50) {
            grade = 'C';
          } 
        } 
        
       
        
        out.println(grade);
      } int m = in.nextInt(); m in.nextInt() in.nextInt in int f = in.nextInt(); f in.nextInt() in.nextInt in int r = in.nextInt(); r in.nextInt() in.nextInt in char grade = 'F'; grade 'F' int tmp = m + f; tmp m + f m f if (m == -1 || f == -1 ) {
          if (m == -1 && f == -1 &&  r == -1) {
            break;
          } 
        } else if (tmp >= 80) {
          grade = 'A';
        } else if (tmp >= 65) {
          grade = 'B';
        } else if (tmp >= 50) {
          grade = 'C';
        } else if (tmp >= 30) {
          grade = 'D';
          
          if (r >= 50) {
            grade = 'C';
          } 
        } m == -1 || f == -1 m == -1 m -1 1 f == -1 f -1 1 {
          if (m == -1 && f == -1 &&  r == -1) {
            break;
          } 
        } 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;
          } break; if (tmp >= 80) {
          grade = 'A';
        } else if (tmp >= 65) {
          grade = 'B';
        } else if (tmp >= 50) {
          grade = 'C';
        } else if (tmp >= 30) {
          grade = 'D';
          
          if (r >= 50) {
            grade = 'C';
          } 
        } tmp >= 80 tmp 80 {
          grade = 'A';
        } grade = 'A' grade 'A' if (tmp >= 65) {
          grade = 'B';
        } else if (tmp >= 50) {
          grade = 'C';
        } else if (tmp >= 30) {
          grade = 'D';
          
          if (r >= 50) {
            grade = 'C';
          } 
        } tmp >= 65 tmp 65 {
          grade = 'B';
        } grade = 'B' grade 'B' if (tmp >= 50) {
          grade = 'C';
        } else if (tmp >= 30) {
          grade = 'D';
          
          if (r >= 50) {
            grade = 'C';
          } 
        } tmp >= 50 tmp 50 {
          grade = 'C';
        } grade = 'C' grade 'C' if (tmp >= 30) {
          grade = 'D';
          
          if (r >= 50) {
            grade = 'C';
          } 
        } tmp >= 30 tmp 30 {
          grade = 'D';
          
          if (r >= 50) {
            grade = 'C';
          } 
        } grade = 'D' grade 'D' if (r >= 50) {
            grade = 'C';
          } r >= 50 r 50 {
            grade = 'C';
          } grade = 'C' grade 'C' out.println(grade) out.println out grade static class InputReader {
    public BufferedReader br;
    public StringTokenizer st;

    public InputReader(InputStream stream) {
      br = new BufferedReader(new InputStreamReader(stream));
    }

    public int nextInt() throws IOException {
      while (st == null || !st.hasMoreTokens()) {
        st = new StringTokenizer(br.readLine());
      }
      return Integer.parseInt(st.nextToken());
    }

    public long nextLong() throws IOException {
      while (st == null || !st.hasMoreTokens()) {
        st = new StringTokenizer(br.readLine());
      }
      return Long.parseLong(st.nextToken());
    }

    public String next() throws IOException {
      while (st == null || !st.hasMoreTokens()) {
        st = new StringTokenizer(br.readLine());
      }
      return st.nextToken();
    }

    public double nextDouble() throws IOException {
      while (st == null || !st.hasMoreTokens()) {
        st = new StringTokenizer(br.readLine());
      }
      return Double.parseDouble(st.nextToken());
    }


  } InputReader public BufferedReader br; br public StringTokenizer st; st public InputReader(InputStream stream) {
      br = new BufferedReader(new InputStreamReader(stream));
    } InputReader {
      br = new BufferedReader(new InputStreamReader(stream));
    } br = new BufferedReader(new InputStreamReader(stream)) br new BufferedReader(new InputStreamReader(stream)) InputStream stream stream public int nextInt() throws IOException {
      while (st == null || !st.hasMoreTokens()) {
        st = new StringTokenizer(br.readLine());
      }
      return Integer.parseInt(st.nextToken());
    } nextInt {
      while (st == null || !st.hasMoreTokens()) {
        st = new StringTokenizer(br.readLine());
      }
      return Integer.parseInt(st.nextToken());
    } while (st == null || !st.hasMoreTokens()) {
        st = new StringTokenizer(br.readLine());
      } st == null || !st.hasMoreTokens() st == null st null !st.hasMoreTokens() st.hasMoreTokens() st.hasMoreTokens st {
        st = new StringTokenizer(br.readLine());
      } st = new StringTokenizer(br.readLine()) st new StringTokenizer(br.readLine()) return Integer.parseInt(st.nextToken()); Integer.parseInt(st.nextToken()) Integer.parseInt Integer st.nextToken() st.nextToken st public long nextLong() throws IOException {
      while (st == null || !st.hasMoreTokens()) {
        st = new StringTokenizer(br.readLine());
      }
      return Long.parseLong(st.nextToken());
    } nextLong {
      while (st == null || !st.hasMoreTokens()) {
        st = new StringTokenizer(br.readLine());
      }
      return Long.parseLong(st.nextToken());
    } while (st == null || !st.hasMoreTokens()) {
        st = new StringTokenizer(br.readLine());
      } st == null || !st.hasMoreTokens() st == null st null !st.hasMoreTokens() st.hasMoreTokens() st.hasMoreTokens st {
        st = new StringTokenizer(br.readLine());
      } st = new StringTokenizer(br.readLine()) st new StringTokenizer(br.readLine()) return Long.parseLong(st.nextToken()); Long.parseLong(st.nextToken()) Long.parseLong Long st.nextToken() st.nextToken st public String next() throws IOException {
      while (st == null || !st.hasMoreTokens()) {
        st = new StringTokenizer(br.readLine());
      }
      return st.nextToken();
    } next {
      while (st == null || !st.hasMoreTokens()) {
        st = new StringTokenizer(br.readLine());
      }
      return st.nextToken();
    } while (st == null || !st.hasMoreTokens()) {
        st = new StringTokenizer(br.readLine());
      } st == null || !st.hasMoreTokens() st == null st null !st.hasMoreTokens() st.hasMoreTokens() st.hasMoreTokens st {
        st = new StringTokenizer(br.readLine());
      } st = new StringTokenizer(br.readLine()) st new StringTokenizer(br.readLine()) return st.nextToken(); st.nextToken() st.nextToken st public double nextDouble() throws IOException {
      while (st == null || !st.hasMoreTokens()) {
        st = new StringTokenizer(br.readLine());
      }
      return Double.parseDouble(st.nextToken());
    } nextDouble {
      while (st == null || !st.hasMoreTokens()) {
        st = new StringTokenizer(br.readLine());
      }
      return Double.parseDouble(st.nextToken());
    } while (st == null || !st.hasMoreTokens()) {
        st = new StringTokenizer(br.readLine());
      } st == null || !st.hasMoreTokens() st == null st null !st.hasMoreTokens() st.hasMoreTokens() st.hasMoreTokens st {
        st = new StringTokenizer(br.readLine());
      } st = new StringTokenizer(br.readLine()) st new StringTokenizer(br.readLine()) return Double.parseDouble(st.nextToken()); Double.parseDouble(st.nextToken()) Double.parseDouble Double st.nextToken() st.nextToken st public class Main {
  static InputReader in;
  static PrintWriter out;

  static class Solution {
    void solve() throws IOException {
      while (true) {
        int m = in.nextInt();
        int f = in.nextInt();
        int r = in.nextInt();
        char grade = 'F';
        int tmp = m + f;
        
        if (m == -1 || f == -1 ) {
          if (m == -1 && f == -1 &&  r == -1) {
            break;
          } 
        } else if (tmp >= 80) {
          grade = 'A';
        } else if (tmp >= 65) {
          grade = 'B';
        } else if (tmp >= 50) {
          grade = 'C';
        } else if (tmp >= 30) {
          grade = 'D';
          
          if (r >= 50) {
            grade = 'C';
          } 
        } 
        
       
        
        out.println(grade);
      }
      
//      End solve()
    }
//    End Solution
}

  public static void main(String[] args) throws IOException {
    in = new InputReader(System.in);
    out = new PrintWriter(System.out);
    Solution solver = new Solution();
    solver.solve();
    out.close();
}

  static class InputReader {
    public BufferedReader br;
    public StringTokenizer st;

    public InputReader(InputStream stream) {
      br = new BufferedReader(new InputStreamReader(stream));
    }

    public int nextInt() throws IOException {
      while (st == null || !st.hasMoreTokens()) {
        st = new StringTokenizer(br.readLine());
      }
      return Integer.parseInt(st.nextToken());
    }

    public long nextLong() throws IOException {
      while (st == null || !st.hasMoreTokens()) {
        st = new StringTokenizer(br.readLine());
      }
      return Long.parseLong(st.nextToken());
    }

    public String next() throws IOException {
      while (st == null || !st.hasMoreTokens()) {
        st = new StringTokenizer(br.readLine());
      }
      return st.nextToken();
    }

    public double nextDouble() throws IOException {
      while (st == null || !st.hasMoreTokens()) {
        st = new StringTokenizer(br.readLine());
      }
      return Double.parseDouble(st.nextToken());
    }


  }

} public class Main {
  static InputReader in;
  static PrintWriter out;

  static class Solution {
    void solve() throws IOException {
      while (true) {
        int m = in.nextInt();
        int f = in.nextInt();
        int r = in.nextInt();
        char grade = 'F';
        int tmp = m + f;
        
        if (m == -1 || f == -1 ) {
          if (m == -1 && f == -1 &&  r == -1) {
            break;
          } 
        } else if (tmp >= 80) {
          grade = 'A';
        } else if (tmp >= 65) {
          grade = 'B';
        } else if (tmp >= 50) {
          grade = 'C';
        } else if (tmp >= 30) {
          grade = 'D';
          
          if (r >= 50) {
            grade = 'C';
          } 
        } 
        
       
        
        out.println(grade);
      }
      
//      End solve()
    }
//    End Solution
}

  public static void main(String[] args) throws IOException {
    in = new InputReader(System.in);
    out = new PrintWriter(System.out);
    Solution solver = new Solution();
    solver.solve();
    out.close();
}

  static class InputReader {
    public BufferedReader br;
    public StringTokenizer st;

    public InputReader(InputStream stream) {
      br = new BufferedReader(new InputStreamReader(stream));
    }

    public int nextInt() throws IOException {
      while (st == null || !st.hasMoreTokens()) {
        st = new StringTokenizer(br.readLine());
      }
      return Integer.parseInt(st.nextToken());
    }

    public long nextLong() throws IOException {
      while (st == null || !st.hasMoreTokens()) {
        st = new StringTokenizer(br.readLine());
      }
      return Long.parseLong(st.nextToken());
    }

    public String next() throws IOException {
      while (st == null || !st.hasMoreTokens()) {
        st = new StringTokenizer(br.readLine());
      }
      return st.nextToken();
    }

    public double nextDouble() throws IOException {
      while (st == null || !st.hasMoreTokens()) {
        st = new StringTokenizer(br.readLine());
      }
      return Double.parseDouble(st.nextToken());
    }


  }

} Main
import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStream;
import java.io.InputStreamReader;
import java.io.PrintWriter;
import java.util.StringTokenizer;

public class Main {
  static InputReader in;
  static PrintWriter out;

  static class Solution {
    void solve() throws IOException {
      while (true) {
        int m = in.nextInt();
        int f = in.nextInt();
        int r = in.nextInt();
        char grade = 'F';
        int tmp = m + f;
        
        if (m == -1 || f == -1 ) {
          if (m == -1 && f == -1 &&  r == -1) {
            break;
          } 
        } else if (tmp >= 80) {
          grade = 'A';
        } else if (tmp >= 65) {
          grade = 'B';
        } else if (tmp >= 50) {
          grade = 'C';
        } else if (tmp >= 30) {
          grade = 'D';
          
          if (r >= 50) {
            grade = 'C';
          } 
        } 
        
       
        
        out.println(grade);
      }
      
//      End solve()
    }
//    End Solution
}

  public static void main(String[] args) throws IOException {
    in = new InputReader(System.in);
    out = new PrintWriter(System.out);
    Solution solver = new Solution();
    solver.solve();
    out.close();
}

  static class InputReader {
    public BufferedReader br;
    public StringTokenizer st;

    public InputReader(InputStream stream) {
      br = new BufferedReader(new InputStreamReader(stream));
    }

    public int nextInt() throws IOException {
      while (st == null || !st.hasMoreTokens()) {
        st = new StringTokenizer(br.readLine());
      }
      return Integer.parseInt(st.nextToken());
    }

    public long nextLong() throws IOException {
      while (st == null || !st.hasMoreTokens()) {
        st = new StringTokenizer(br.readLine());
      }
      return Long.parseLong(st.nextToken());
    }

    public String next() throws IOException {
      while (st == null || !st.hasMoreTokens()) {
        st = new StringTokenizer(br.readLine());
      }
      return st.nextToken();
    }

    public double nextDouble() throws IOException {
      while (st == null || !st.hasMoreTokens()) {
        st = new StringTokenizer(br.readLine());
      }
      return Double.parseDouble(st.nextToken());
    }


  }

}