array = gets.split("").map(&:to_i)
n=array.length
if n ==1
  puts 0
else
i=0
even=[]
odd=[]
while i<n
  if i%2 == 0
    even.push(array[i])
  else
    odd.push(array[i])
  end
  i+=1
end
eb=even.inject(:+)
ob=odd.inject(:+)
ew=even.length - eb
ow=odd.length - ob
if ew+ob > ow+eb
  puts ow+eb
else
  puts ew+ob
end
end