if __name__ == "__main__":
    apples = int(input(), 2)
    score = 0
    while apples != 0:
        t = apples & 3
        if t == 1:
            score += 1
            apples >>= 2
        elif t == 3:
            score += 1
            apples += 1
        elif t == 0:
            apples >>= 2
        else:
            apples >>= 1
    print(score)