import pypyjit
pypyjit.set_param("max_unroll_recursion=-1")
import sys
sys.setrecursionlimit(10**7)
from collections import deque
S = input()

stack = []

def f():
    global ans
    ans += 1
    for _ in range(3):stack.pop()
    stack.append("0")
    if(len(stack) > 2):
        if(stack[-3:] == ["1","1","0"]):f()
    stack.append("1")
    stack.append("1")

ans = 0
for i in range(len(S)):
    stack.append(S[i])
    if(len(stack) <= 2):continue
    if(stack[-3:] == ["1","1","0"]):
        f()
print(ans)