public static void main ( String [ ] args ) {
  Scanner sc = new Scanner ( System . in ) ;
  int len = sc . nextInt ( ) ;
  int jump = sc . nextInt ( ) ;
  String str = sc . next ( ) ;
  int pos = 0 ;
  int ans = 0 ;
  while ( pos < len - 1 ) {
    int i = jump ;
    if ( pos + i > len - 1 ) {
      i = len - 1 - pos ;
    }
    while ( str . charAt ( pos + i ) == '0' ) {
      i -- ;
    }
    if ( i == 0 ) {
      ans = - 1 ;
      break ;
    }
    ans ++ ;
    pos += i ;
  }
  System . out . println ( ans ) ;
}

public static void main ( String [ ] args ) { Scanner sc = new Scanner ( System . in ) ; int len = sc . nextInt ( ) ; int jump = sc . nextInt ( ) ; String str = sc . next ( ) ; int pos = 0 ; int ans = 0 ; while ( pos < len - 1 ) { int i = jump ; if ( pos + i > len - 1 ) { i = len - 1 - pos ; } while ( str . charAt ( pos + i ) == '0' ) { i -- ; } if ( i == 0 ) { ans = - 1 ; break ; } ans ++ ; pos += i ; } System . out . println ( ans ) ; }
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