public static void main ( String [ ] args ) {
  Scanner in = new Scanner ( System . in ) ;
  int a = in . nextInt ( ) , b = in . nextInt ( ) , n = in . nextInt ( ) , i ;
  double o = Double . MAX_VALUE ;
  for ( i = 0 ;
  i < n ;
  i ++ ) {
    int x = in . nextInt ( ) , y = in . nextInt ( ) , v = in . nextInt ( ) ;
    double d = Math . sqrt ( Math . pow ( x - a , 2 ) + Math . pow ( y - b , 2 ) ) ;
    o = Math . min ( o , d / v ) ;
  }
  in . close ( ) ;
  System . out . println ( o ) ;
}

public static void main ( String [ ] args ) { Scanner in = new Scanner ( System . in ) ; int a = in . nextInt ( ) , b = in . nextInt ( ) , n = in . nextInt ( ) , i ; double o = Double . MAX_VALUE ; for ( i = 0 ; i < n ; i ++ ) { int x = in . nextInt ( ) , y = in . nextInt ( ) , v = in . nextInt ( ) ; double d = Math . sqrt ( Math . pow ( x - a , 2 ) + Math . pow ( y - b , 2 ) ) ; o = Math . min ( o , d / v ) ; } in . close ( ) ; System . out . println ( o ) ; }
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