import sys
from collections import deque


# sys.setrecursionlimit(10**6)


def debug(*args):
    print(*args, file=sys.stderr)


s = deque(list(map(int, list(input()))))
# stack = []
stack_count = 0
ans = 0
while s:
    si = s.popleft()
    if si == 0:
        p = stack_count // 2
        ans += p
        stack_count = 2*p
        # next_stack = []
        # while len(stack) >= 2 and stack[-2] == stack[-1] == 1:
        #     ans += 1
        #     next_stack.append(stack.pop())
        #     next_stack.append(stack.pop())
        # stack, next_stack = next_stack, stack
    else:
        # stack.append(1)
        stack_count += 1

print(ans)