import Control.Applicative
import Data.List
import Data.Array.Unboxed
import qualified Data.Sequence
import qualified Data.Set
import Data.Maybe
import Debug.Trace

-- 2-stack queue
-- partial persistent として使うのであれば各操作 amortized O(1)
data Queue a = Node [a] [a]

queueEmpty = Node [] []

queueNull (Node [] []) = True
queueNull (Node _ _) = False

enqueue x (Node s1 s2) = Node s1 (x : s2)

dequeue (Node [] s2) = dequeue (Node (reverse s2) [])
dequeue (Node (s:s1) s2) = (s, Node s1 s2)

main = do
    [h, w] <- map read . words <$> getLine :: IO [Int]
    ss <- concat . map (\(i, s) -> zip (zip (repeat i) [1..]) s) . zip [1..] . lines <$> getContents :: IO [((Int, Int), Char)]
    let grid = array ((1, 1), (h, w)) ss :: UArray (Int, Int) Char
    let (y, x) = startPoint grid
    print $ bfs grid h w (enqueue (y, x, 0, 0) queueEmpty) (Data.Set.fromList [(y, x, 0)])

startPoint = fst . fromJust . find (\(i, x) -> x == 'S') . Data.Array.Unboxed.assocs

bfs :: UArray (Int, Int) Char -> Int -> Int -> Queue (Int, Int, Int, Int) -> Data.Set.Set (Int, Int, Int) -> Int
bfs grid h w q vis 
    | queueNull q          = -1
    | grid ! (y, x) == 'G' = turn
    | otherwise            = 
        let
            inside y x = 1 <= y && y <= h && 1 <= x && x <= w
            dd = if t == 0 then
                    [(1, 2), (1, -2), (-1, 2), (-1, -2), (2, 1), (2, -1), (-2, 1), (-2, -1)]
                 else
                    [(-1, -1), (-1, 1), (1, -1), (1, 1)]
            npos = [(y + dy, x + dx) | (dy, dx) <- dd, inside (y + dy) (x + dx)]
            nst = [(ny, nx, nt) | (ny, nx) <- npos, nt <- [if grid ! (ny, nx) == 'R' then 1 - t else t], Data.Set.notMember (ny, nx, nt) vis]
            nstate = [(ny, nx, nt, turn + 1) | (ny, nx, nt) <- nst]
            nvis = foldr Data.Set.insert vis nst
            nque = foldr enqueue que nstate
        in bfs grid h w nque nvis
    where
        ((y, x, t, turn), que) = dequeue q