def readints():
  return list(map(int, input().split()))

def deep_recursion():
  import sys
  import pypyjit  # type: ignore
  sys.setrecursionlimit(550000)
  pypyjit.set_param('max_unroll_recursion=-1')

def main():
  S = map(int, input())
  ans = 0
  ones = 0
  for d in S:
    if d == 0:
      ans += ones // 2
      ones = ones // 2 * 2
    else:
      ones += 1
  print(ans)


if __name__ == '__main__':
  # deep_recursion()
  main()