import sys
from collections import defaultdict, Counter, deque
from itertools import permutations, combinations, product, combinations_with_replacement, groupby, accumulate
import operator
from math import sqrt, gcd, factorial
# from math import isqrt, prod, comb  #python3.8用(notpypy)
#from bisect import bisect_left, bisect_right
#from functools import lru_cache, reduce
#from heapq import heappush, heappop, heapify, heappushpop, heapreplace
#import numpy as np
#import networkx as nx
#from networkx.utils import UnionFind
#from numba import njit, b1, i1, i4, i8, f8
# numba例 @njit(i1(i4[:], i8[:, :]),cache=True) 引数i4配列、i8 2次元配列,戻り値i1
#from scipy.sparse import csr_matrix
#from scipy.sparse.csgraph import shortest_path, floyd_warshall, dijkstra, bellman_ford, johnson, NegativeCycleError, maximum_bipartite_matching, maximum_flow, minimum_spanning_tree
def input(): return sys.stdin.readline().rstrip()
def divceil(n, k): return 1+(n-1)//k  # n/kの切り上げを返す
def yn(hantei, yes='Yes', no='No'): print(yes if hantei else no)


def main():
    mod = 10**9+7
    mod2 = 998244353
    n = int(input())
    s = input()
    dp = defaultdict(int)
    bet = 'abcdefghijklmnopqrstuvwxyz'
    if n == 1 and s == '?':
        print(26)
    elif n == 1:
        print(1)
    else:
        if s[:2] == '??':
            for a, b in product(bet, repeat=2):
                if a != b:
                    dp[a+b] = 1
        elif s[0] == '?':
            for a in bet:
                if a != s[1]:
                    dp[a+s[1]] = 1
        elif s[1] == '?':
            for a in bet:
                if a != s[0]:
                    dp[s[0]+a] = 1
        elif s[0] != s[1]:
            dp[s[:2]] = 1
        for ss in s[2:]:
            newdp = defaultdict(int)
            if ss == '?':
                for key, value in dp.items():
                    v = value % mod2
                    for a in bet:
                        if key[0] != a and key[1] != a:
                            newdp[key[1]+a] += v
            else:
                for key, value in dp.items():
                    v = value % mod2
                    if key[0] != ss and key[1] != ss:
                        newdp[key[1]+ss] += v
            dp=newdp
        print(sum(v for v in dp.values())%mod2)


if __name__ == '__main__':
    main()