S=list(map(int, input()))
S.append(2)
i = 2
ans=0
while i < len(S):
	if S[i] == 0 and S[i-1] == S[i-2] == 1:
		S[i-2] = 0
		S[i] = 1
		i = max(2,i-2)
		ans += 1
		continue
	i += 1
print(ans)