3651: 「BalticOI 2011 Day1」打开灯泡 Switch the Lamp On

**译自 BalticOI 2011 Day1 T3「Switch the Lamp On」** 有一种正方形的电路元件，在它的两组相对顶点中，有一组会用导线连接起来，另一组则不会。 有 $N\times M$ 个这样的元件，你想将其排列成 $N$ 行 $M$ 列放在电路板上。电路板的左上角连接电源，右下角连接灯泡。  试求：至少要旋转多少个正方形元件才能让电源与灯泡连通，若无解则输出 $\texttt{NO SOLUTION}$。 Casper is designing an electronic circuit on a $N ×M$ rectangular grid plate. There are $N ×M$ square tiles that are aligned to the grid on the plate. Two (out of four) opposite corners of each tile are connected by a wire. A power source is connected to the top left corner of the plate. A lamp is connected to the bottom right corner of the plate. The lamp is on only if there is a path of wires connecting power source to lamp. In order to switch the lamp on, any number of tiles can be turned by $90°$ (in both directions). In the picture above the lamp is off. If any one of the tiles in the second column from the right is turned by $90°$ , power source and lamp get connected, and the lamp is on. Write a program to find out the minimal number of tiles that have to be turned by $90°$ to switch the lamp on.

第一行有两个整数 $N$ 和 $M$。 在接下来的 $N$ 行中，每行有 $M$ 个字符。每个字符均为 `\` 或 `/`，表示正方形元件上导线的连接方向。 The first line of input contains two integer numbers $N$ and $M$, the dimensions of the plate. In each of the following $N$ lines there are $M$ symbols – either `\` or `/` – which indicate the direction of the wire connecting the opposite vertices of the corresponding tile.

输出共一行，若有解则输出一个整数，表示至少要旋转多少个正方形元件才能让电源与灯泡连通；若无解则输出 `NO SOLUTION`。 There must be exactly one line of output. If it is possible to switch the lamp on, this line must contain only one integer number: the minimal number of tiles that have to be turned to switch on the lamp. If it is not possible, output the string: `NO SOLUTION`

3 5
\\/\\
\\///
/\\\\

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