local mfl, mce = math.floor, math.ceil
local mod = 998244353

local n = io.read("*n", "*l")
local s = io.read()
local t = {}
for i = 1, n do
  t[i] = s:sub(i, i)
end
for i = 1, n - 1 do
  if t[i] == t[i + 1] and t[i] ~= "?" then
    print(0) os.exit()
  end
end
for i = 1, n - 2 do
  if t[i] == t[i + 2] and t[i] ~= "?" then
    print(0) os.exit()
  end
end
if n == 1 then
  print(t[1] == "?" and 26 or 1) os.exit()
end
local dp1, dp2 = {}, {}
if t[1] == "?" and t[2] == "?" then
  for i = 1, 26 do for j = 1, 26 do
    local v = (i - 1) * 26 + j
    dp1[v] = i == j and 0 or 1
  end end
elseif t[1] == "?" then
  local i2 = t[2]:byte() - 96
  for i = 1, 26 do for j = 1, 26 do
    local v = (i - 1) * 26 + j
    if i == i2 or j ~= i2 then
      dp1[v] = 0
    else
      dp1[v] = 1
    end
  end end
elseif t[2] == "?" then
  local i1 = t[1]:byte() - 96
  for i = 1, 26 do for j = 1, 26 do
    local v = (i - 1) * 26 + j
    if i ~= i1 or j == i1 then
      dp1[v] = 0
    else
      dp1[v] = 1
    end
  end end
else
  local i1 = t[1]:byte() - 96
  local i2 = t[2]:byte() - 96
  for i = 1, 26 do for j = 1, 26 do
    local v = (i - 1) * 26 + j
    if i ~= i1 or j ~= i2 then
      dp1[v] = 0
    else
      dp1[v] = 1
    end
  end end
end
for i = 3, n do
  local src = i % 2 == 1 and dp1 or dp2
  local dst = i % 2 == 1 and dp2 or dp1
  for j = 1, 676 do dst[j] = 0 end
  for prv_state = 1, 676 do
    local srcval = src[prv_state]
    local prvprv = mce(prv_state / 26)
    local prv = prv_state - (prvprv - 1) * 26
    if t[i] == "?" then
      for j = 1, 26 do
        if j ~= prvprv and j ~= prv then
          local d = (prv - 1) * 26 + j
          dst[d] = dst[d] + srcval
        end
      end
    else
      local z = t[i]:byte() - 96
      for j = 1, 26 do
        if j == z and j ~= prvprv and j ~= prv then
          local d = (prv - 1) * 26 + j
          dst[d] = dst[d] + srcval
        end
      end
    end
  end
  for j = 1, 676 do dst[j] = dst[j] % mod end
end
local tbl = n % 2 == 1 and dp2 or dp1
local ret = 0
for i = 1, 676 do
  ret = ret + tbl[i]
end
print(ret % mod)
-- print(os.clock())