S = gets.chomp.chars

color = '0'
count1 = 0
S.each do |c|
  count1 += 1 if c != color
  color = color == '0' ? '1' : '0'
end

color = '1'
count2 = 0
S.each do |c|
  count2 += 1 if c != color
  color = color == '0' ? '1' : '0'
end

ans = [count1, count2].min
puts ans
