CF1204A.BowWow and the Timetable

In the city of Saint Petersburg, a day lasts for $2^{100}$ minutes. From the main station of Saint Petersburg, a train departs after $1$ minute, $4$ minutes, $16$ minutes, and so on; in other words, the train departs at time $4^k$ for each integer $k \geq 0$. Team BowWow has arrived at the station at the time $s$ and it is trying to count how many trains have they missed; in other words, the number of trains that have departed strictly before time $s$. For example if $s = 20$, then they missed trains which have departed at $1$, $4$ and $16$. As you are the only one who knows the time, help them!

Note that the number $s$ will be given you in a binary representation without leading zeroes.

在圣彼得堡市，一天持续 $2^{100}$ 分钟。一列火车从圣彼得堡主站出发，出发时刻依次为第 $1$ 分钟、第 $4$ 分钟、第 $16$ 分钟，依此类推；换言之，对每个整数 $k \geq 0$，火车在时刻 $4^k$ 出发。BowWow 小队于时刻 $s$ 抵达车站，他们正试图计算自己已错过的列车数量；即，在严格早于时刻 $s$ 的时间已出发的列车数目。例如，若 $s = 20$，则他们已错过出发时刻为 $1$、$4$ 和 $16$ 的列车。作为唯一知晓当前时间的人，请帮助他们！

注意：数字 $s$ 将以二进制表示法（不含前导零）的形式给出。二进制表示法

The first line contains a single binary number $s$ ($0 \leq s \lt 2^{100}$) without leading zeroes.

第一行包含一个二进制数 $s$（$0 \leq s \lt 2^{100}$），不含前导零。

Output a single number — the number of trains which have departed strictly before the time $s$.

输出一个整数——在时间 $s$ 之前严格出发的列车数量。

In the first example $100000000_2 = 256_{10}$, missed trains have departed at $1$, $4$, $16$ and $64$.

In the second example $101_2 = 5_{10}$, trains have departed at $1$ and $4$.

The third example is explained in the statements.

在第一个例子中，$100000000_2 = 256_{10}$，错过的列车发车时刻为 $1$、$4$、$16$ 和 $64$。

在第二个例子中，$101_2 = 5_{10}$，列车发车时刻为 $1$ 和 $4$。

第三个例子已在题目描述中说明。