import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;
 
public class Main {

  public static void main(String[] args) throws IOException {
    try ( BufferedReader br = new BufferedReader(new InputStreamReader(System.in)) ) {

      String[] lines;
      int m,f,r,sum;

      do {

        lines = br.readLine().split(" ");
        m = Integer.parseInt(lines[0]);
        f = Integer.parseInt(lines[1]);
        r = Integer.parseInt(lines[2]);
        sum = m + f;

        if ( m == -1 && f == -1 && r == -1) break;
        else if( m == -1 || f == -1 ) {
          System.out.println("F");
          continue;
        }

        if ( sum >= 80 ) System.out.println("A");
        else if ( sum >= 65 ) System.out.println("B");
        else if ( sum >= 50 ) System.out.println("C");
        else if ( sum >= 30 ) {
          if ( r >= 50 ) System.out.println("C");
          else System.out.println("D");
        } else System.out.println("F");
      } while( br.ready() );
    }
  }
} import java.io.BufferedReader; BufferedReader java.io import java.io.IOException; IOException java.io import java.io.InputStreamReader; InputStreamReader java.io public class Main {

  public static void main(String[] args) throws IOException {
    try ( BufferedReader br = new BufferedReader(new InputStreamReader(System.in)) ) {

      String[] lines;
      int m,f,r,sum;

      do {

        lines = br.readLine().split(" ");
        m = Integer.parseInt(lines[0]);
        f = Integer.parseInt(lines[1]);
        r = Integer.parseInt(lines[2]);
        sum = m + f;

        if ( m == -1 && f == -1 && r == -1) break;
        else if( m == -1 || f == -1 ) {
          System.out.println("F");
          continue;
        }

        if ( sum >= 80 ) System.out.println("A");
        else if ( sum >= 65 ) System.out.println("B");
        else if ( sum >= 50 ) System.out.println("C");
        else if ( sum >= 30 ) {
          if ( r >= 50 ) System.out.println("C");
          else System.out.println("D");
        } else System.out.println("F");
      } while( br.ready() );
    }
  }
} Main public static void main(String[] args) throws IOException {
    try ( BufferedReader br = new BufferedReader(new InputStreamReader(System.in)) ) {

      String[] lines;
      int m,f,r,sum;

      do {

        lines = br.readLine().split(" ");
        m = Integer.parseInt(lines[0]);
        f = Integer.parseInt(lines[1]);
        r = Integer.parseInt(lines[2]);
        sum = m + f;

        if ( m == -1 && f == -1 && r == -1) break;
        else if( m == -1 || f == -1 ) {
          System.out.println("F");
          continue;
        }

        if ( sum >= 80 ) System.out.println("A");
        else if ( sum >= 65 ) System.out.println("B");
        else if ( sum >= 50 ) System.out.println("C");
        else if ( sum >= 30 ) {
          if ( r >= 50 ) System.out.println("C");
          else System.out.println("D");
        } else System.out.println("F");
      } while( br.ready() );
    }
  } main {
    try ( BufferedReader br = new BufferedReader(new InputStreamReader(System.in)) ) {

      String[] lines;
      int m,f,r,sum;

      do {

        lines = br.readLine().split(" ");
        m = Integer.parseInt(lines[0]);
        f = Integer.parseInt(lines[1]);
        r = Integer.parseInt(lines[2]);
        sum = m + f;

        if ( m == -1 && f == -1 && r == -1) break;
        else if( m == -1 || f == -1 ) {
          System.out.println("F");
          continue;
        }

        if ( sum >= 80 ) System.out.println("A");
        else if ( sum >= 65 ) System.out.println("B");
        else if ( sum >= 50 ) System.out.println("C");
        else if ( sum >= 30 ) {
          if ( r >= 50 ) System.out.println("C");
          else System.out.println("D");
        } else System.out.println("F");
      } while( br.ready() );
    }
  } try ( BufferedReader br = new BufferedReader(new InputStreamReader(System.in)) ) {

      String[] lines;
      int m,f,r,sum;

      do {

        lines = br.readLine().split(" ");
        m = Integer.parseInt(lines[0]);
        f = Integer.parseInt(lines[1]);
        r = Integer.parseInt(lines[2]);
        sum = m + f;

        if ( m == -1 && f == -1 && r == -1) break;
        else if( m == -1 || f == -1 ) {
          System.out.println("F");
          continue;
        }

        if ( sum >= 80 ) System.out.println("A");
        else if ( sum >= 65 ) System.out.println("B");
        else if ( sum >= 50 ) System.out.println("C");
        else if ( sum >= 30 ) {
          if ( r >= 50 ) System.out.println("C");
          else System.out.println("D");
        } else System.out.println("F");
      } while( br.ready() );
    } {

      String[] lines;
      int m,f,r,sum;

      do {

        lines = br.readLine().split(" ");
        m = Integer.parseInt(lines[0]);
        f = Integer.parseInt(lines[1]);
        r = Integer.parseInt(lines[2]);
        sum = m + f;

        if ( m == -1 && f == -1 && r == -1) break;
        else if( m == -1 || f == -1 ) {
          System.out.println("F");
          continue;
        }

        if ( sum >= 80 ) System.out.println("A");
        else if ( sum >= 65 ) System.out.println("B");
        else if ( sum >= 50 ) System.out.println("C");
        else if ( sum >= 30 ) {
          if ( r >= 50 ) System.out.println("C");
          else System.out.println("D");
        } else System.out.println("F");
      } while( br.ready() );
    } BufferedReader br = new BufferedReader(new InputStreamReader(System.in)) BufferedReader br = new BufferedReader(new InputStreamReader(System.in)) new BufferedReader(new InputStreamReader(System.in)) String[] lines; lines int m m f f r r sum; sum do {

        lines = br.readLine().split(" ");
        m = Integer.parseInt(lines[0]);
        f = Integer.parseInt(lines[1]);
        r = Integer.parseInt(lines[2]);
        sum = m + f;

        if ( m == -1 && f == -1 && r == -1) break;
        else if( m == -1 || f == -1 ) {
          System.out.println("F");
          continue;
        }

        if ( sum >= 80 ) System.out.println("A");
        else if ( sum >= 65 ) System.out.println("B");
        else if ( sum >= 50 ) System.out.println("C");
        else if ( sum >= 30 ) {
          if ( r >= 50 ) System.out.println("C");
          else System.out.println("D");
        } else System.out.println("F");
      } while( br.ready() ); br.ready() br.ready br {

        lines = br.readLine().split(" ");
        m = Integer.parseInt(lines[0]);
        f = Integer.parseInt(lines[1]);
        r = Integer.parseInt(lines[2]);
        sum = m + f;

        if ( m == -1 && f == -1 && r == -1) break;
        else if( m == -1 || f == -1 ) {
          System.out.println("F");
          continue;
        }

        if ( sum >= 80 ) System.out.println("A");
        else if ( sum >= 65 ) System.out.println("B");
        else if ( sum >= 50 ) System.out.println("C");
        else if ( sum >= 30 ) {
          if ( r >= 50 ) System.out.println("C");
          else System.out.println("D");
        } else System.out.println("F");
      } lines = br.readLine().split(" ") lines br.readLine().split(" ") br.readLine().split br.readLine() br.readLine br " " m = Integer.parseInt(lines[0]) m Integer.parseInt(lines[0]) Integer.parseInt Integer lines[0] lines 0 f = Integer.parseInt(lines[1]) f Integer.parseInt(lines[1]) Integer.parseInt Integer lines[1] lines 1 r = Integer.parseInt(lines[2]) r Integer.parseInt(lines[2]) Integer.parseInt Integer lines[2] lines 2 sum = m + f sum m + f m f if ( m == -1 && f == -1 && r == -1) break;
        else if( m == -1 || f == -1 ) {
          System.out.println("F");
          continue;
        } 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");
          continue;
        } m == -1 || f == -1 m == -1 m -1 1 f == -1 f -1 1 {
          System.out.println("F");
          continue;
        } System.out.println("F") System.out.println System.out System System.out "F" continue; if ( sum >= 80 ) System.out.println("A");
        else if ( sum >= 65 ) System.out.println("B");
        else if ( sum >= 50 ) System.out.println("C");
        else if ( sum >= 30 ) {
          if ( r >= 50 ) System.out.println("C");
          else System.out.println("D");
        } else System.out.println("F"); sum >= 80 sum 80 System.out.println("A") System.out.println System.out System System.out "A" if ( sum >= 65 ) System.out.println("B");
        else if ( sum >= 50 ) System.out.println("C");
        else if ( sum >= 30 ) {
          if ( r >= 50 ) System.out.println("C");
          else System.out.println("D");
        } else System.out.println("F"); sum >= 65 sum 65 System.out.println("B") System.out.println System.out System System.out "B" if ( sum >= 50 ) System.out.println("C");
        else if ( sum >= 30 ) {
          if ( r >= 50 ) System.out.println("C");
          else System.out.println("D");
        } else System.out.println("F"); sum >= 50 sum 50 System.out.println("C") System.out.println System.out System System.out "C" if ( sum >= 30 ) {
          if ( r >= 50 ) System.out.println("C");
          else System.out.println("D");
        } else System.out.println("F"); sum >= 30 sum 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 System.out System System.out "C" System.out.println("D") System.out.println System.out System System.out "D" 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) throws IOException {
    try ( BufferedReader br = new BufferedReader(new InputStreamReader(System.in)) ) {

      String[] lines;
      int m,f,r,sum;

      do {

        lines = br.readLine().split(" ");
        m = Integer.parseInt(lines[0]);
        f = Integer.parseInt(lines[1]);
        r = Integer.parseInt(lines[2]);
        sum = m + f;

        if ( m == -1 && f == -1 && r == -1) break;
        else if( m == -1 || f == -1 ) {
          System.out.println("F");
          continue;
        }

        if ( sum >= 80 ) System.out.println("A");
        else if ( sum >= 65 ) System.out.println("B");
        else if ( sum >= 50 ) System.out.println("C");
        else if ( sum >= 30 ) {
          if ( r >= 50 ) System.out.println("C");
          else System.out.println("D");
        } else System.out.println("F");
      } while( br.ready() );
    }
  }
} public class Main {

  public static void main(String[] args) throws IOException {
    try ( BufferedReader br = new BufferedReader(new InputStreamReader(System.in)) ) {

      String[] lines;
      int m,f,r,sum;

      do {

        lines = br.readLine().split(" ");
        m = Integer.parseInt(lines[0]);
        f = Integer.parseInt(lines[1]);
        r = Integer.parseInt(lines[2]);
        sum = m + f;

        if ( m == -1 && f == -1 && r == -1) break;
        else if( m == -1 || f == -1 ) {
          System.out.println("F");
          continue;
        }

        if ( sum >= 80 ) System.out.println("A");
        else if ( sum >= 65 ) System.out.println("B");
        else if ( sum >= 50 ) System.out.println("C");
        else if ( sum >= 30 ) {
          if ( r >= 50 ) System.out.println("C");
          else System.out.println("D");
        } else System.out.println("F");
      } while( br.ready() );
    }
  }
} Main
import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;
 
public class Main {

  public static void main(String[] args) throws IOException {
    try ( BufferedReader br = new BufferedReader(new InputStreamReader(System.in)) ) {

      String[] lines;
      int m,f,r,sum;

      do {

        lines = br.readLine().split(" ");
        m = Integer.parseInt(lines[0]);
        f = Integer.parseInt(lines[1]);
        r = Integer.parseInt(lines[2]);
        sum = m + f;

        if ( m == -1 && f == -1 && r == -1) break;
        else if( m == -1 || f == -1 ) {
          System.out.println("F");
          continue;
        }

        if ( sum >= 80 ) System.out.println("A");
        else if ( sum >= 65 ) System.out.println("B");
        else if ( sum >= 50 ) System.out.println("C");
        else if ( sum >= 30 ) {
          if ( r >= 50 ) System.out.println("C");
          else System.out.println("D");
        } else System.out.println("F");
      } while( br.ready() );
    }
  }
}
