题目描述

Wowo is a solo adventurer who completed many dangerous journeys on his own foot in forests, deserts and even glaciers. The Shanghai ICPC (Shanghai Invitational Contest on Programmable Cheating) committee invited Wowo as a tester of their new running trial.

The trial can be described as a 2D simple polyline $(p_1,\ldots, p_n)$. In other words, the trial consists of $n-1$ line segments $(p_1, p_2),\ldots, (p_{n-1}, p_n)$. The line segments do not intersect with each other except that two consecutive line segments $(p_i, p_{i+1})$ and $(p_{i+1}, p_{i+2})$ intersect at the point $p_{i+1}$. Any two consecutive segments have different directions. The committee wants Wowo to run from $p_1$ to $p_n$ along the line segments $(p_1,p_2),\ldots, (p_{n-1}, p_n)$ in order.

However, Wowo has a smart device that can hack the committee's system for an interval of time. Wowo is able to choose $2$ points $a, b$ on the trial and run directly from $a$ to $b$ along the line segment $(a, b)$. Each of these $a$ and $b$ can be some $p_i$ ($1\le i\le n$) and can be some point on some line segment $(p_i, p_{i+1})$ ($1\le i<n$) as well. Before reaching $a$ and after reaching $b$, Wowo has to run along the original trial. Wowo does not want to be caught cheating, so he decided that the line segment $(a, b)$ should not intersect or touch any line segment of the trial at any point other than $a$ and $b$. Help Wowo to choose $a$ and $b$ wisely and output the shortest distance Wowo need to run from $p_1$ to $p_n$ using his smart cheating device.

输入格式

The first line includes a single integer $n$ indicating the number of points on the running trial ($2\le n\le 200$).

The $i+1$-th line ($1\le i\le n$) contains two integers $x$ and $y$ separated by a single space indicating the coordinates of $p_i$ ($-1000\le x, y\le 1000$).

It is guaranteed that the line segments do not intersect with each other except that two consecutive line segments $(p_i, p_{i+1})$ and $(p_{i+1}, p_{i+2})$ intersect at the point $p_{i+1}$. In other words, $(p_i, p_{i+1})\cap (p_{j}, p_{j+1})=\left\{\begin{array}{cc}\emptyset & i\neq j-1\\ \{p_{j}\} & i = j-1\end{array}\right.$ holds for any integers $i, j$ satisfying $1\le i< j<n$. Here $(s, t)$ represents all points on the line segment from $s$ to $t$ including $s$ and $t$.

It is guaranteed that each line segment has nonzero length. In other words, $p_i\neq p_{i+1}$ for any integer $i\in [1, n)$.

It is guaranteed that adjacent line segments are not collinear. In other words, for any integer $i\in [1,n-2]$ and any real number $\lambda$, $p_i - p_{i+1}$ is $\textbf{not}$ equal to $\lambda(p_{i+1}-p_{i+2})$.

输出格式

Output the shortest distance Wowo needs to run. Your answer will be considered correct if its absolute or relative error does not exceed $10^{-6}$.

5
0 0
1 10
2 0
3 10
4 0

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