def solve(io)
  n = io.get
  s = io.get(String)

  io.put_e 1 if n == 1

  dp = Array.new_md(n + 1, 26, 26, Mint.new(0))
  (0...26).each do |j|
    (0...26).each do |k|
      if (s[0] == '?' || s[0] - 'a' == j) && (s[1] == '?' || s[1] - 'a' == k) && j != k
        dp[2][j][k] = Mint.new(1)
      end
    end
  end

  (2...n).each do |i|
    (0...26).each do |j|
      next unless s[i] == '?' || s[i] - 'a' == j
      c = (0...26).map { |k| (0...26).map { |l| dp[i][l][k] }.sum }
      (0...26).each do |k|
        next if j == k
        dp[i + 1][k][j] += c[k] - dp[i][j][k]
      end
    end
  end

  io.put dp[n].map(&.sum).sum
end

class ProconIO
  def initialize(@ins : IO = STDIN, @outs : IO = STDOUT)
    @buf = IO::Memory.new("")
  end

  def get(k : T.class = Int32) forall T
    get_v(k)
  end

  macro define_get
    {% for i in (2..9) %}
      def get({{ *(1..i).map { |j| "k#{j}".id } }})
        { {{ *(1..i).map { |j| "get(k#{j})".id } }} }
      end
    {% end %}
  end
  define_get

  macro define_getn
    {% for i in (2..9) %}
      def get{{i}}(k : T.class = Int32) forall T
        get({{ *(1..i).map { "k".id } }})
      end
    {% end %}
  end
  define_getn

  def get_a(n : Int, k : T.class = Int32) forall T
    Array.new(n) { get_v(k) }
  end

  def get_c(n : Int, k : T.class = Int32) forall T
    get_a(n, k)
  end

  macro define_get_c
    {% for i in (2..9) %}
      def get_c(n : Int, {{ *(1..i).map { |j| "k#{j}".id } }})
        a = Array.new(n) { get({{ *(1..i).map { |j| "k#{j}".id } }}) }
        { {{ *(1..i).map { |j| "a.map { |e| e[#{j-1}] }".id } }} }
      end
    {% end %}
  end
  define_get_c

  macro define_getn_c
    {% for i in (2..9) %}
      def get{{i}}_c(n : Int, k : T.class = Int32) forall T
        get_c(n, {{ *(1..i).map { "k".id } }})
      end
    {% end %}
  end
  define_getn_c

  def get_m(r : Int, c : Int, k : T.class = Int32) forall T
    Array.new(r) { get_a(c, k) }
  end

  macro define_put
    {% for i in (1..9) %}
      def put({{ *(1..i).map { |j| "v#{j}".id } }}, *, delimiter = " ")
        {% for j in (1..i) %}
          print_v(v{{j}}, delimiter)
          {% if j < i %}@outs << delimiter{% end %}
        {% end %}
        @outs.puts
      end
    {% end %}
  end
  define_put

  def put_e(*vs)
    put(*vs)
    exit
  end

  def put_f(*vs)
    put(*vs)
    @outs.flush
  end


  private def get_v(k : Int32.class); get_token.to_i32; end
  private def get_v(k : UInt32.class); get_token.to_u32; end
  private def get_v(k : Int64.class); get_token.to_i64; end
  private def get_v(k : UInt64.class); get_token.to_u64; end
  private def get_v(k : Float64.class); get_token.to_f64; end
  private def get_v(k : String.class); get_token; end

  private def get_token
    loop do
      token = @buf.gets(' ', chomp: true)
      break token unless token.nil?
      @buf = IO::Memory.new(@ins.read_line)
    end
  end

  private def print_v(v, dlimiter)
    @outs << v
  end

  private def print_v(v : Enumerable, delimiter)
    v.each_with_index do |e, i|
      @outs << e
      @outs << delimiter if i < v.size - 1
    end
  end
end

struct Int
  def cdiv(b : Int)
    (self + b - 1) // b
  end

  def bit?(i : Int)
    bit(i) == 1
  end

  def set_bit(i : Int)
    self | (self.class.new(1) << i)
  end

  def reset_bit(i : Int)
    self & ~(self.class.new(1) << i)
  end

  
  {% if compare_versions(env("CRYSTAL_VERSION") || "0.0.0", "0.35.0") < 0 %}
    def digits(base = 10)
      raise ArgumentError.new("Invalid base #{base}") if base < 2
      raise ArgumentError.new("Can't request digits of negative number") if self < 0
      return [0] if self == 0

      num = self
      digits_count = (Math.log(self.to_f + 1) / Math.log(base)).ceil.to_i
      ary = Array(Int32).new(digits_count)
      while num != 0
        ary << num.remainder(base).to_i
        num = num.tdiv(base)
      end
      ary
    end
  {% end %}

  {% if compare_versions(env("CRYSTAL_VERSION") || "0.0.0", "0.34.0") < 0 %}
    def bit_length : Int32
      x = self < 0 ? ~self : self

      if x.is_a?(Int::Primitive)
        Int32.new(sizeof(self) * 8 - x.leading_zeros_count)
      else
        to_s(2).size
      end
    end
  {% end %}
end

struct Float64
  def near?(x)
    (self - x).abs <= (self.abs < x.abs ? x.abs : self.abs) * EPSILON
  end
end

struct Number
  {% if compare_versions(env("CRYSTAL_VERSION") || "0.0.0", "1.1.0") < 0 %}
    def zero?
      self == 0
    end

    def positive?
      self > 0
    end

    def negative?
      self < 0
    end
  {% end %}

  {% if compare_versions(env("CRYSTAL_VERSION") || "0.0.0", "0.36.0") < 0 %}
    def self.additive_identity
      zero
    end

    def self.multiplicative_identity
      new(1)
    end
  {% end %}
end

class Array
  macro new_md(*args, &block)
    {% if !block %}
      {% for arg, i in args[0...-2] %}
        Array.new({{arg}}) {
      {% end %}
      Array.new({{args[-2]}}, {{args[-1]}})
      {% for arg in args[0...-2] %}
        }
      {% end %}
    {% else %}
      {% for arg, i in args %}
        Array.new({{arg}}) { |_i{{i}}|
      {% end %}
      {% for block_arg, i in block.args %}
        {{block_arg}} = _i{{i}}
      {% end %}
      {{block.body}}
      {% for arg in args %}
        }
      {% end %}
    {% end %}
  end
end

module Math
  {% if compare_versions(env("CRYSTAL_VERSION") || "0.0.0", "1.2.0") < 0 %}
    def isqrt(value : Int::Primitive)
      raise ArgumentError.new "Input must be non-negative integer" if value < 0
      return value if value < 2
      res = value.class.zero
      bit = res.succ << (res.leading_zeros_count - 2)
      bit >>= value.leading_zeros_count & ~0x3
      while (bit != 0)
        if value >= res + bit
          value -= res + bit
          res = (res >> 1) + bit
        else
          res >>= 1
        end
        bit >>= 2
      end
      res
    end
  {% end %}
end

macro min_u(a, b)
  {{a}} = Math.min({{a}}, {{b}})
end

macro max_u(a, b)
  {{a}} = Math.max({{a}}, {{b}})
end

macro zip(a, *b, &block)
  {{a}}.zip({{*b}}) {{block}}
end

def powr(a : T, n : Int, i : T = T.multiplicative_identity) forall T
  powr(a, n, i) { |a, b| a * b }
end

def powr(a : T, n : Int, i : T = T.multiplicative_identity, &block) forall T
  return i if n == 0
  r, b = i, a
  while n > 0
    r = yield r, b if n.bit(0) == 1
    b = yield b, b
    n >>= 1
  end
  r
end

def ext_gcd(a : T, b : T) forall T
  if a == 0
    {b, T.new(0), T.new(1)}
  else
    g, x, y = ext_gcd(b % a, a)
    {g, y - (b // a) * x, x}
  end
end

def bit_subsets(a : Int, includes_zero = false)
  n = i = a
  if includes_zero
    while i >= 0
      yield i & n
      i = (i & n) - 1
    end
  else
    while i > 0
      yield i
      i = (i - 1) & n
    end
  end
end

def bit_zeta_trans_subset(n : Int, f : Array(T), &compose : (T, T) -> T) forall T
  g = Array.new(1 << n) { |i| f[i] }
  n.times do |i|
    (1 << n).times do |j|
      if j >> i & 1 != 0
        g[j] = compose.call(g[j], g[j ^ (1 << i)])
      end
    end
  end
  g
end

def bit_zeta_trans_superset(n : Int, f : Array(T), &compose : (T, T) -> T) forall T
  g = Array.new(1 << n) { |i| f[i] }
  n.times do |i|
    (1 << n).times do |j|
      if j >> i & 1 == 0
        g[j] = compose.call(g[j], g[j ^ (1 << i)])
      end
    end
  end
  g
end

abstract struct ModInt < Number
  macro new_type(name, mod)
    struct {{name}} < ModInt
      @@mod : Int32 = {{mod}}
    end
  end

  def initialize(v : Int)
    @v = (v % @@mod).to_i64
  end

  def_hash @@mod, @v

  def to_s
    @v.to_s
  end

  def to_s(io : IO) : Nil
    @v.to_s(io)
  end

  getter v : Int64

  delegate to_i, to: @v

  def ==(r : self)
    @v == r.v
  end

  def ==(r : Int)
    @v == (r % @@mod)
  end

  def - : self
    m(-@v)
  end

  def +(r : self)
    m(@v + r.v)
  end

  def +(r : Int)
    self + m(r)
  end

  def -(r : self)
    m(@v - r.v)
  end

  def -(r : Int)
    self - m(r)
  end

  def *(r : self)
    m(@v * r.v)
  end

  def *(r : Int)
    self * m(r)
  end

  def //(r : self)
    self * r.inv
  end

  def //(r : Int)
    self // m(r)
  end

  def **(n : Int)
    powr(self, n)
  end

  def inv
    m(ext_gcd(@v.to_i32, @@mod)[1])
  end

  def self.array(v : Array(Int))
    v.map { |vi| new(vi) }
  end

  def self.array(v : Array(Array(Int)))
    v.map { |vi| vi.map { |vij| new(vij) } }
  end


  private def m(v : Int)
    self.class.new(v)
  end
end
ModInt.new_type(Mint, 998244353)

solve(ProconIO.new)