public static void main ( String [ ] args ) throws Exception {
  Scanner in = new Scanner ( new File ( "Main.in" ) ) ;
  PrintWriter out = new PrintWriter ( new File ( "Main.out" ) ) ;
  int tests = in . nextInt ( ) ;
  for ( int test = 1 ;
  test <= tests ;
  test ++ ) {
    out . print ( "Case #" + test + ": " ) ;
    int n = in . nextInt ( ) ;
    int k = in . nextInt ( ) ;
    int B = in . nextInt ( ) ;
    int T = in . nextInt ( ) ;
    int [ ] x = new int [ n ] ;
    int [ ] v = new int [ n ] ;
    for ( int i = 0 ;
    i < n ;
    i ++ ) x [ i ] = in . nextInt ( ) ;
    for ( int i = 0 ;
    i < n ;
    i ++ ) v [ i ] = in . nextInt ( ) ;
    boolean [ ] can = new boolean [ n ] ;
    int z = 0 ;
    for ( int i = 0 ;
    i < n ;
    i ++ ) {
      can [ i ] = x [ i ] + v [ i ] * T >= B ;
      if ( can [ i ] ) z ++ ;
    }
    if ( z < k ) {
      out . println ( "IMPOSSIBLE" ) ;
      continue ;
    }
    int ans = 0 ;
    z = 0 ;
    for ( int i = n - 1 ;
    i >= 0 && k > 0 ;
    i -- ) if ( can [ i ] ) {
      ans += z ;
      k -- ;
    }
    else z ++ ;
    out . println ( ans ) ;
  }
  out . close ( ) ;
}

public static void main ( String [ ] args ) throws Exception { Scanner in = new Scanner ( new File ( " Main . in " ) ) ; PrintWriter out = new PrintWriter ( new File ( " Main . out " ) ) ; int tests = in . nextInt ( ) ; for ( int test = 1 ; test <= tests ; test ++ ) { out . print ( " Case ▁ # " + test + " : ▁ " ) ; int n = in . nextInt ( ) ; int k = in . nextInt ( ) ; int B = in . nextInt ( ) ; int T = in . nextInt ( ) ; int [ ] x = new int [ n ] ; int [ ] v = new int [ n ] ; for ( int i = 0 ; i < n ; i ++ ) x [ i ] = in . nextInt ( ) ; for ( int i = 0 ; i < n ; i ++ ) v [ i ] = in . nextInt ( ) ; boolean [ ] can = new boolean [ n ] ; int z = 0 ; for ( int i = 0 ; i < n ; i ++ ) { can [ i ] = x [ i ] + v [ i ] * T >= B ; if ( can [ i ] ) z ++ ; } if ( z < k ) { out . println ( " IMPOSSIBLE " ) ; continue ; } int ans = 0 ; z = 0 ; for ( int i = n - 1 ; i >= 0 && k > 0 ; i -- ) if ( can [ i ] ) { ans += z ; k -- ; } else z ++ ; out . println ( ans ) ; } out . close ( ) ; }
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