给定一个大小为N*M的迷宫。迷宫有通道和墙壁组成，每一步可以向邻接的上下左右的通道移动。请求出从起点到终点所需要的最小步数。(Given a maze of N*M size. The labyrinth is composed of channels and walls, and each step can move to the adjacent, upper and lower channels. The minimum number of steps required from the starting point to the end is requested.)