public static void main ( String [ ] args ) {
  Scanner sc = new Scanner ( System . in ) ;
  int total = sc . nextInt ( ) ;
  int ride = sc . nextInt ( ) ;
  int a = sc . nextInt ( ) ;
  int b = sc . nextInt ( ) ;
  int ans = total * a ;
  int count = ride ;
  while ( count <= total ) {
    ans = Math . min ( ans , count / ride * b + ( total - count ) * a ) ;
    count += ride ;
  }
  System . out . println ( Math . min ( total % ride == 0 ? total / ride * b : ( total / ride + 1 ) * b , ans ) ) ;
}

public static void main ( String [ ] args ) { Scanner sc = new Scanner ( System . in ) ; int total = sc . nextInt ( ) ; int ride = sc . nextInt ( ) ; int a = sc . nextInt ( ) ; int b = sc . nextInt ( ) ; int ans = total * a ; int count = ride ; while ( count <= total ) { ans = Math . min ( ans , count / ride * b + ( total - count ) * a ) ; count += ride ; } System . out . println ( Math . min ( total % ride == 0 ? total / ride * b : ( total / ride + 1 ) * b , ans ) ) ; }
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