public static void main ( String [ ] args ) throws Exception {
  BufferedReader reader = new BufferedReader ( new InputStreamReader ( System . in ) ) ;
  PrintWriter writer = new PrintWriter ( System . out ) ;
  StringTokenizer info = new StringTokenizer ( reader . readLine ( ) ) ;
  int n = Integer . parseInt ( info . nextToken ( ) ) ;
  int k = Integer . parseInt ( info . nextToken ( ) ) ;
  int sum = 0 ;
  StringTokenizer aT = new StringTokenizer ( reader . readLine ( ) ) ;
  int [ ] a = new int [ n ] ;
  int count = 0 ;
  for ( int i = 0 ;
  i < n ;
  i ++ ) {
    a [ i ] = Integer . parseInt ( aT . nextToken ( ) ) ;
    sum += a [ i ] ;
    int out = Math . min ( sum , 8 ) ;
    k -= out ;
    sum -= out ;
    count ++ ;
    if ( k <= 0 ) {
      break ;
    }
  }
  if ( k > 0 ) {
    writer . println ( "-1" ) ;
  }
  else {
    writer . println ( count ) ;
  }
  reader . close ( ) ;
  writer . close ( ) ;
}

public static void main ( String [ ] args ) throws Exception { BufferedReader reader = new BufferedReader ( new InputStreamReader ( System . in ) ) ; PrintWriter writer = new PrintWriter ( System . out ) ; StringTokenizer info = new StringTokenizer ( reader . readLine ( ) ) ; int n = Integer . parseInt ( info . nextToken ( ) ) ; int k = Integer . parseInt ( info . nextToken ( ) ) ; int sum = 0 ; StringTokenizer aT = new StringTokenizer ( reader . readLine ( ) ) ; int [ ] a = new int [ n ] ; int count = 0 ; for ( int i = 0 ; i < n ; i ++ ) { a [ i ] = Integer . parseInt ( aT . nextToken ( ) ) ; sum += a [ i ] ; int out = Math . min ( sum , 8 ) ; k -= out ; sum -= out ; count ++ ; if ( k <= 0 ) { break ; } } if ( k > 0 ) { writer . println ( " - 1" ) ; } else { writer . println ( count ) ; } reader . close ( ) ; writer . close ( ) ; }
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