import java.io.BufferedReader
import java.io.InputStream
import java.io.InputStreamReader
import java.lang.AssertionError
import java.util.*
import kotlin.math.abs
import kotlin.math.max
import kotlin.math.min

val MOD = 998244353

fun powMod(a: Long, n: Long, mod: Int): Long {
  if (n == 0L) return 1
  val res = powMod(a * a % mod, n / 2, mod)
  return if (n % 2 == 1L) res * a % mod else res
}
class Solver(stream: InputStream, private val out: java.io.PrintWriter) {
  private val reader = BufferedReader(InputStreamReader(stream), 32768)

  fun solve() {
    val N = ni()
    val S = ns()

    val R = 28
    var cur = Array(2){LongArray(R*R)}
    var next = Array(2){LongArray(R*R)}
    cur[0][26*R + 27] = 1
    
    fun countPal(): Long {
      for (i in 0 until N) {
        next[0].fill(0)
        next[1][0] = 0

        val sum1 = LongArray(R)
        val sum2 = LongArray(R)
        for (c1 in 0 until R) {
          for (c2 in 0 until R) {
            val s = c1 * R + c2
            sum1[c1] += cur[0][s]
            sum2[c2] += cur[0][s]
          }
        }
        debug{"i:$i"}
        debug(sum1)
        debug(sum2)

        fun doIt(c3: Int) {
          next[1][0] += sum1[c3] + sum2[c3]
          for (c2 in 0 until R) {
            if (c2 == c3) continue
            next[0][c2*R + c3] += sum2[c2] + MOD - (cur[0][c3*R+c2])%MOD
          }
        }

        if (S[i] == '?') {
          for (c3 in 0 until 26) {
            doIt(c3)
            next[1][0] += cur[1][0]
          }
        }
        else {
          val c3 = S[i]-'a'
          doIt(c3)
          next[1][0] += cur[1][0]
        }

        for (c1 in 0 until R) {
          for (c2 in 0 until R) {
            val s = c1*R + c2
            next[0][s] = next[0][s] % MOD
            next[1][0] = next[1][0] % MOD
          }
        }

        val tmp = cur
        cur = next
        next = tmp
      }

      return cur[1][0]
    }

    val M = S.count{it == '?'}
    val ans = (powMod(26, M.toLong(), MOD) + (MOD - countPal())) % MOD
    out.println(ans)
  }













































  private val isDebug = try {
    // なんか本番でエラーでる
    System.getenv("MY_DEBUG") != null
  } catch (t: Throwable) {
    false
  }

  private var tokenizer: StringTokenizer? = null
  private fun next(): String {
    while (tokenizer == null || !tokenizer!!.hasMoreTokens()) {
      tokenizer = StringTokenizer(reader.readLine())
    }
    return tokenizer!!.nextToken()
  }

  private fun ni() = next().toInt()
  private fun nl() = next().toLong()
  private fun ns() = next()
  private fun na(n: Int, offset: Int = 0): IntArray {
    return IntArray(n) { ni() + offset }
  }
  private fun nal(n: Int, offset: Int = 0): LongArray {
    val res = LongArray(n)
    for (i in 0 until n) {
      res[i] = nl() + offset
    }
    return res
  }

  private fun na2(n: Int, offset: Int = 0): Array<IntArray> {
    val a  = Array(2){IntArray(n)}
    for (i in 0 until n) {
      for (e in a) {
        e[i] = ni() + offset
      }
    }
    return a
  }

  private inline fun debug(msg: () -> String) {
    if (isDebug) System.err.println(msg())
  }

  /**
   * コーナーケースでエラー出たりするので、debug(dp[1])のように添え字付きの場合はdebug{}をつかうこと
   */
  private inline fun debug(a: LongArray) {
    debug { a.joinToString(" ") }
  }

  private inline fun debug(a: IntArray) {
    debug { a.joinToString(" ") }
  }

  private inline fun debug(a: BooleanArray) {
    debug { toString(a) }
  }

  private inline fun toString(a: BooleanArray) = run{a.map { if (it) 1 else 0 }.joinToString("")}

  private inline fun debugDim(A: Array<LongArray>) {
    if (isDebug) {
      for (a in A) {
        debug(a)
      }
    }
  }
  private inline fun debugDim(A: Array<IntArray>) {
    if (isDebug) {
      for (a in A) {
        debug(a)
      }
    }
  }
  private inline fun debugDim(A: Array<BooleanArray>) {
    if (isDebug) {
      for (a in A) {
        debug(a)
      }
    }
  }

  /**
   * 勝手にimport消されるのを防ぎたい
   */
  private fun hoge() {
    min(1, 2)
    max(1, 2)
    abs(-10)
  }

  private inline fun assert(b: Boolean) = run{if (!b) throw AssertionError()}
  private inline fun assert(b: Boolean, f: () -> String) = run{if (!b) throw AssertionError(f())}

  companion object {
    // TestRunnerから呼びたいので単純なmainじゃだめ
    fun main() {
      val out = java.io.PrintWriter(System.out)
      Solver(System.`in`, out).solve()
      out.flush()
    }
  }
}

/**
 * judgeから呼ばれる
 */
fun main() = Solver.main()