#include <cassert>
#include <cmath>
#include <algorithm>
#include <iostream>
#include <iomanip>
#include <limits.h>
#include <map>
#include <queue>
#include <set>
#include <string.h>
#include <vector>

using namespace std;
typedef long long ll;

const ll MOD = 998244353;
ll dp[50010][26][26];

int main() {
  int N;
  cin >> N;
  string S;
  cin >> S;

  if (N == 1) {
    if (S[0] == '?') {
      cout << 26 << endl;
    } else {
      cout << 1 << endl;
    }
    return 0;
  }

  memset(dp, 0, sizeof(dp));

  for (int u = 0; u < 26; ++u) {
    if (S[0] != '?' && S[0] - 'a' != u) continue;

    for (int v = 0; v < 26; ++v) {
      if (u == v) continue;
      if (S[1] != '?' && S[1] - 'a' != v) continue;
      dp[1][u][v]++;
    }
  }

  for (int i = 2; i < N; ++i) {
    char s = S[i];
    ll sum[26];
    memset(sum, 0, sizeof(sum));

    for (int u = 0; u < 26; ++u) {
      for (int v = 0; v < 26; ++v) {
        sum[v] += dp[i - 1][u][v];
        sum[v] %= MOD;
      }
    }

    if (s == '?') {
      for (int u = 0; u < 26; ++u) {
        for (int v = 0; v < 26; ++v) {
          if (u == v) continue;

          dp[i][u][v] += (sum[u] - dp[i - 1][v][u] + MOD) % MOD;
          dp[i][u][v] %= MOD;
        }
      }
    } else {
      int v = s - 'a';
      for (int u = 0; u < 26; ++u) {
        if (u == v) continue;
        dp[i][u][v] += (sum[u] - dp[i - 1][v][u] + MOD) % MOD;
        dp[i][u][v] %= MOD;
      }
    }
  }

  ll ans = 0;

  for (int u = 0; u < 26; ++u) {
    for (int v = 0; v < 26; ++v) {
      if (u == v) continue;

      ans += dp[N - 1][u][v];
      ans %= MOD;
    }
  }

  cout << ans << endl;

  return 0;
}