/* -*- coding: utf-8 -*-
 *
 * 1646.cc:  No.1646 Avoid Palindrome - yukicoder
 */

#include<cstdio>
#include<cstdlib>
#include<cstring>
#include<cmath>
#include<iostream>
#include<string>
#include<vector>
#include<map>
#include<set>
#include<stack>
#include<list>
#include<queue>
#include<deque>
#include<algorithm>
#include<numeric>
#include<utility>
#include<complex>
#include<functional>
 
using namespace std;

/* constant */

const int MAX_N = 50000;
const int MOD = 998244353;

/* typedef */

/* global variables */

char s[MAX_N + 4];
int dp[MAX_N + 1][26][26];

/* subroutines */

inline void addmod(int &a, int b) { a = (a + b) % MOD; }

/* main */

int main() {
  int n;
  scanf("%d%s", &n, s);

  if (n == 1) {
    printf("%d\n", (s[0] == '?') ? 26 : 1);
    return 0;
  }

  if (s[0] == '?') {
    for (int s0 = 0; s0 < 26; s0++) {
      if (s[1] == '?')
	for (int s1 = 0; s1 < 26; s1++)
	  dp[2][s0][s1] = (s0 != s1) ? 1 : 0;
      else {
	int s1 = s[1] - 'a';
	dp[2][s0][s1] = (s0 != s1) ? 1 : 0;
      }
    }
  }
  else {
    int s0 = s[0] - 'a';
    if (s[1] == '?')
      for (int s1 = 0; s1 < 26; s1++)
	dp[2][s0][s1] = (s0 != s1) ? 1 : 0;
    else {
      int s1 = s[1] - 'a';
      dp[2][s0][s1] = (s0 != s1) ? 1 : 0;
    }
  }

  for (int i = 2; i < n; i++)
    for (int s0 = 0; s0 < 26; s0++)
      for (int s1 = 0; s1 < 26; s1++)
	if (dp[i][s0][s1] > 0) {
	  if (s[i] == '?')
	    for (int s2 = 0; s2 < 26; s2++) {
	      if (s0 != s2 && s1 != s2)
		addmod(dp[i + 1][s1][s2], dp[i][s0][s1]);
	    }
	  else {
	    int s2 = s[i] - 'a';
	    if (s0 != s2 && s1 != s2)
	      addmod(dp[i + 1][s1][s2], dp[i][s0][s1]);
	  }
	}	  

  int sum = 0;
  for (int s0 = 0; s0 < 26; s0++)
    for (int s1 = 0; s1 < 26; s1++)
      addmod(sum, dp[n][s0][s1]);
  printf("%d\n", sum);
  return 0;
}