program main
  implicit none

  character(len=100000) :: s, s1 , s2
  integer :: n, i, ans1, ans2
  

  read(*,*) s
  s1 = s
  s2 = s
  n = len_trim(s)

  ans1 = 0
  ans2 = 0
  do i = 2, n
     if(s1(i:i) == s1(i-1:i-1)) then
        select case(s1(i:i))
        case('0')
           s1(i:i) = '1'
        case('1')
           s1(i:i) = '0'
        end select
        ans1 = ans1 + 1
     end if

  end do

  do i = 1, n-1
     if(s2(n-i:n-i) == s2(n-i+1:n-i+1)) then
        select case(s2(n-i:n-i))
        case('0')
           s2(n-i:n-i) = '1'
        case('1')
           s2(n-i:n-i) = '0'
        end select
        ans2 = ans2 + 1
     end if
  end do
  
  write(*,*) min(ans1, ans2)

end program main

  
