s=[*input()]
while s and s[-1]=='1':
  s.pop()
ans=z=0
while s:
  while s and s[-1]=='0':
    s.pop()
    z+=1
  while s and s[-2:]==[*'11']:
    ans+=z
    s.pop();s.pop()
  while s and s[-1]=='1':
    z=0
    s.pop()
print(ans)