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)
  if t[i] == "?" then
    t[i] = false
  else
    t[i] = t[i]:byte() - 96
  end
end
for i = 1, n - 1 do
  if t[i] and t[i] == t[i + 1] then
    print(0) os.exit()
  end
end
for i = 1, n - 2 do
  if t[i] and t[i] == t[i + 2] then
    print(0) os.exit()
  end
end
if n == 1 then
  print(t[1] and 1 or 26) os.exit()
end
local dp1, dp2 = {}, {}
if not t[1] and not 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 not t[1] then
  local i2 = t[2]
  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 not t[2] then
  local i1 = t[1]
  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]
  local i2 = t[2]
  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
  if t[i] then
    local z = t[i]
    for prvprv = 1, 26 do
      if prvprv ~= z then
        for prv = 1, 26 do
          if prv ~= z then
            local d = (prv - 1) * 26 + z
            dst[d] = dst[d] + src[(prvprv - 1) * 26 + prv]
          end
        end
      end
    end
  else
    local row = {}
    local col = {}
    for j = 1, 26 do row[j], col[j] = 0, 0 end
    local all = 0
    for j = 1, 26 do
      for k = 1, 26 do
        local sz = src[(j - 1) * 26 + k]
        all = all + sz
        row[j] = row[j] + sz
        col[k] = col[k] + sz
      end
    end
    local z = t[i]
    for j = 1, 26 do
      for prv = 1, 26 do
        if prv ~= j then
          local v = col[prv] - src[(j - 1) * 26 + prv]
          local d = (prv - 1) * 26 + j
          dst[d] = v
        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())