P7733 [JDWOI-2] Rescue Experimental Data

A toxic gas leak occurred in a laboratory of a large experimental center, and the experimenters want to rescue the experimental data.

The experimental center can be regarded as an undirected connected graph with $n$ vertices and $m$ edges. Every second, each experimenter can move to an adjacent laboratory and **collect** the data there, while the toxic gas spreads every second to all adjacent laboratories. When an experimenter **returns to the hall $s$**, we say that they have **rescued** the data. Experimenters cannot enter laboratories filled with toxic gas (and it is also not allowed if they and the gas enter the same laboratory in the same second). **There are strict protection measures around the hall, so it will not be reached by the toxic gas. (See Sample 2 for details.)** Now all experimenters are in the hall $s$, and the laboratory where the gas leaks is vertex $t$. Assuming there are **enough** experimenters to depart at the same time, what is the maximum number of laboratories’ data that can be rescued?

The first line contains two positive integers $n, m$, indicating the number of vertices and edges in the experimental center. Lines $2$ to $m + 1$ each contain two positive integers $u, v$, indicating that there is an edge between laboratories $u$ and $v$. Line $m + 2$ contains two positive integers $s, t$, indicating the hall and the gas leak vertex.

Output one integer in a single line, indicating the maximum number of laboratories whose data can be rescued.

**Please pay attention to the impact of constant factors on runtime efficiency.** [Sample Explanation 1] Only Laboratory $2$ can be reached and then returned from. [Sample Explanation 2] Because the hall is indestructible, Laboratories $5$ and $6$ will be reached by the gas, while Laboratories $2$ and $3$ will never be reached by the gas. [Sample Explanation 3] The vertices that can be rescued are: $2, 3, 4, 5, 11, 12$. [Constraints] **This problem uses bundled testdata.** For $10\%$ of the testdata, $2 \leq n, m \leq 20$; For $30\%$ of the testdata, $2 \leq n \leq 2000, 1 \leq m \leq 10000$; For $70\%$ of the testdata, $2 \leq n \leq 2 \times 10^5$; For $100\%$ of the testdata, $2 \leq n, m \leq 5 \times 10^6$. Since the input size is very large, here is the fast input template used in the std (you need to choose C++11 or above when submitting). ```cpp char gc() { static char now[1 = '0' && c