[USACO14JAN] Building a Ski Course G

Farmer John is helping to turn his large field into a ski course for the upcoming winter Moolympics. The field has dimensions M x N (1 <= M,N <= 100), and its intended final composition is described by an M x N grid of characters like this: RSRSSS RSRSSS RSRSSS Each character describes how the snow in a unit square of the field should be groomed: either 'R' for 'rough' or 'S' for 'smooth' (the Moolympics organizers think that a course is more interesting if it has a mixture of rough and smooth patches). To build the desired course, Farmer John plans to modify his tractor so that it can stamp any B x B patch of the field (B <= M, B <= N) with either entirely smooth snow or entirely rough snow. Since it takes a long time to reset the tractor between each of these stamps, FJ wants to make B as large as possible. With B = 1, he can clearly create the desired ski course by stamping each individual square with either R or S, as intended. However, for larger values of B, it may no longer be possible to create the desired course design. Every unit square of the course must at some point be stamped by FJ's tractor; it cannot be left in its default state. Please help FJ determine the largest possible value of B he can successfully use. 滑雪场的设计图是一个M\*NM x N (1 <= M,N <= 100)的矩阵，每个格子里用一个字母R（表示粗糙）或者S（表示平整）。 比如： RSRSSS RSRSSS RSRSSS 农民约翰的拖拉机每次可以将一块B\*B (B <= M, B <= N)的区域全部标记B\*B (B <= M, B <= N)的R或者S，他希望B能够尽量地大。一个格子可以被多次标记，下一次标记能够覆盖前一次标记，每个格子可以都至少被标记一次。

\* Line 1: Two space-separated integers M and N. \* Lines 2..M+1: M lines of exactly N characters (each R or S), describing the desired ski course design.

\* Line 1: The maximum value of B Farmer John can use to create the desired course pattern.

3 6 
RSRSSS 
RSRSSS 
RSRSSS 

3 

FJ can stamp a rough patch spanning columns 1-3, followed by a smooth patch spanning columns 2-4, then a rough patch spanning columns 3-5, and finally a smooth patch spanning columns 4-6.