public static void main ( String [ ] args ) {
  Scanner sc = new Scanner ( System . in ) ;
  int n = Integer . parseInt ( sc . nextLine ( ) ) ;
  String rLine = sc . nextLine ( ) ;
  char [ ] r = rLine . toCharArray ( ) ;
  HashMap < Character , Integer > map = new HashMap < Character , Integer > ( ) ;
  map . put ( 'A' , 4 ) ;
  map . put ( 'B' , 3 ) ;
  map . put ( 'C' , 2 ) ;
  map . put ( 'D' , 1 ) ;
  map . put ( 'F' , 0 ) ;
  double sum = 0 ;
  for ( char c : r ) {
    sum += map . get ( c ) ;
  }
  System . out . println ( sum / n ) ;
}

public static void main ( String [ ] args ) { Scanner sc = new Scanner ( System . in ) ; int n = Integer . parseInt ( sc . nextLine ( ) ) ; String rLine = sc . nextLine ( ) ; char [ ] r = rLine . toCharArray ( ) ; HashMap < Character , Integer > map = new HashMap < Character , Integer > ( ) ; map . put ( ' A ' , 4 ) ; map . put ( ' B ' , 3 ) ; map . put ( ' C ' , 2 ) ; map . put ( ' D ' , 1 ) ; map . put ( ' F ' , 0 ) ; double sum = 0 ; for ( char c : r ) { sum += map . get ( c ) ; } System . out . println ( sum / n ) ; }
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