local h, w = io.read("*n", "*n", "*l")
local map = {}
local inf = 1000000007
local len = {}
local spos, gpos = 0, 0
for i = 1, h do
  local s = io.read()
  for j = 1, w do
    local ss = s:sub(j, j)
    local idx = (i - 1) * w + j
    map[idx] = false
    if ss == "S" then
      spos = idx
    elseif ss == "G" then
      gpos = idx
    elseif ss == "R" then
      map[idx] = true
    end
  end
end
for i = 1, h * w * 2 do len[i] = inf end

local taskstate = {}
for i = 1, h * w * 2 do taskstate[i] = false end
local tasks = {}
local tasknum = 0
local done = 0
local tasklim = h * w * 2

local function addtask(mixidx)
  if not taskstate[mixidx] then
    taskstate[mixidx] = true
    tasknum = tasknum + 1
    local taskidx = tasknum % tasklim
    if taskidx == 0 then taskidx = tasklim end
    tasks[taskidx] = mixidx
  end
end

local function walk(src, dst, knight)
  local mixsrc = src
  if knight then
    mixsrc = mixsrc + h * w
  end
  local mixdst = dst
  if knight and not map[dst] then
    mixdst = mixdst + h * w
  elseif not knight and map[dst] then
    mixdst = mixdst + h * w
  end
  if len[mixsrc] + 1 < len[mixdst] then
    len[mixdst] = len[mixsrc] + 1
    addtask(mixdst)
  end
end

len[spos + h * w] = 0
addtask(spos + h * w)

while done < tasknum do
  done = done + 1
  local taskidx = done % tasklim
  if taskidx == 0 then taskidx = tasklim end
  local mixidx = tasks[taskidx]
  taskstate[mixidx] = false
  local knight = h * w < mixidx
  local idx = mixidx
  if knight then idx = idx - h * w end
  if knight then
    if 2 * w < idx then
      if 1 < w then
        if idx % w ~= 0 then walk(idx, idx + 1 - w - w, true) end
        if idx % w ~= 1 then walk(idx, idx - 1 - w - w, true) end
      end
    end
    if idx <= (h - 2) * w then
      if 1 < w then
        if idx % w ~= 0 then walk(idx, idx + 1 + w + w, true) end
        if idx % w ~= 1 then walk(idx, idx - 1 + w + w, true) end
      end
    end
    if w < idx then
      if 2 < w then
        if idx % w ~= 0 and idx % w ~= (w - 1) then
          walk(idx, idx + 2 - w, true)
        end
        if idx % w ~= 1 and idx % w ~= 2 then
          walk(idx, idx - 2 - w, true)
        end
      end
    end
    if idx <= (h - 1) * w then
      if 2 < w then
        if idx % w ~= 0 and idx % w ~= (w - 1) then
          walk(idx, idx + 2 + w, true)
        end
        if idx % w ~= 1 and idx % w ~= 2 then
          walk(idx, idx - 2 + w, true)
        end
      end
    end
  else
    if w < idx then
      if 1 < w then
        if idx % w ~= 0 then walk(idx, idx + 1 - w, false) end
        if idx % w ~= 1 then walk(idx, idx - 1 - w, false) end
      end
    end
    if idx <= (h - 1) * w then
      if 1 < w then
        if idx % w ~= 0 then walk(idx, idx + 1 + w, false) end
        if idx % w ~= 1 then walk(idx, idx - 1 + w, false) end
      end
    end
  end
end
local ret = math.min(len[gpos], len[gpos + h * w])
print(inf <= ret and -1 or ret)