Problem:
Score : 300 points

Problem Statement

N tiles are arranged in a row from left to right. The initial color of each tile is represented by a string S of length N.

The i-th tile from the left is painted black if the i-th character of S is 0, and painted white if that character is 1.

You want to repaint some of the tiles black or white, so that any two adjacent tiles have different colors.

At least how many tiles need to be repainted to satisfy the condition?

Constraints

1 \leq |S| \leq 10^5

S_i is 0 or 1.

Input

Input is given from Standard Input in the following format:

S

Output

Print the minimum number of tiles that need to be repainted to satisfy the condition.

Sample Input 1

000

Sample Output 1

1

The condition can be satisfied by repainting the middle tile white.

Sample Input 2

10010010

Sample Output 2

3

Sample Input 3

0

Sample Output 3

0

Solution in Fortran (gfortran v4.8.4) (judge verdict: Wrong Answer)

module ABC124

  ! <module>s to import
  use, intrinsic :: iso_fortran_env

  ! require all variables to be explicitly declared
  implicit none

  ! accessibility of <subroutine>s and <function>s in this <module>
  public :: task_C

  ! constants for this <module>
  integer(kind=INT32), parameter, private :: len_string_max = 10000_INT32

  ! variables for this <module>
  character(len=len_string_max, kind=1) :: given_string
  character(len=len_string_max, kind=1) :: reference_0, reference_1
  integer  (kind=INT32)                 :: len_string
  integer  (kind=INT32)                 :: num_operations(0:1)

  ! contained <subroutine>s and <function>s are below
  contains

  subroutine task_C

    ! variables for this <subroutine>
    integer(kind=INT32) :: itr

    ! STEP.01
    ! read out the given string
    read(unit=INPUT_UNIT, fmt='(A)') given_string

    ! STEP.02
    ! count up the number of operations
    len_string     = len_trim(given_string)
    num_operations = 0_INT32

    do itr = 1_INT32, len_string, 2_INT32
      reference_0(itr:itr) = '0'
      reference_1(itr:itr) = '1'
    end do

    do itr = 2_INT32, len_string, 2_INT32
      reference_0(itr:itr) = '1'
      reference_1(itr:itr) = '0'
    end do

    do itr = 1_INT32, len_string, 1_INT32
      if (given_string(itr:itr) .ne. reference_0(itr:itr)) num_operations(0) = num_operations(0) + 1_INT32
      if (given_string(itr:itr) .ne. reference_1(itr:itr)) num_operations(1) = num_operations(1) + 1_INT32
    end do

    ! STEP.04
    ! output the result
    write(unit=OUTPUT_UNIT, fmt='(I0)', advance='yes') min(num_operations(0), num_operations(1))

    ! STEP.END
    return

  end subroutine task_C

end module ABC124


program main

  ! <module>s to import
  use, non_intrinsic :: ABC124

  ! require all variables to be explicitly declared
  implicit none

  call task_C

end program main