S = list(input())
one = zero = 0
ans = 0
while S:
    s = S.pop()
    if s == '1':
        one += 1
        if one == 2:
            ans += zero
            one = 0
    else:
        if one > 0:
            one = 0
            zero = 1
        else:
            zero += 1
print(ans)