from collections import deque
q=list(str(input()))
q=deque(list(map(int,q)))

ans=0
s=deque()

while q:
  x=q.popleft()
  s.append(x)
  if 3<=len(s):
    if s[-3]==1 and s[-2]==1 and s[-1]==0:
      s[-3]=0
      s[-1]=1
      ans+=1

while s:
  x=s.pop()
  q.appendleft(x)
  if 3<=len(q):
    if q[0]==1 and q[1]==1 and q[2]==0:
      q[0]=0
      q[2]=1
      ans+=1
print(ans)