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()
    #s = '?'*(5*10**4)
    s = [ord(ss)-97 for ss in s]
    dp = [0]*(26**2)
    bet = list(range(26))
    if n == 1 and s == [-34]:
        print(26)
    elif n == 1:
        print(1)
    else:
        if s[:2] == [-34, -34]:
            for a, b in product(bet, repeat=2):
                if a != b:
                    dp[a*26+b] = 1
        elif s[0] == -34:
            for a in bet:
                if a != s[1]:
                    dp[a*26+s[1]] = 1
        elif s[1] == -34:
            for a in bet:
                if a != s[0]:
                    dp[s[0]*26+a] = 1
        elif s[0] != s[1]:
            dp[s[0]*26+s[1]] = 1
        for ss in s[2:]:
            newdp = [0]*(26**2)
            dpsum= [sum(dp[a*26+b] for a in bet)%mod2 for b in bet]
            if ss == -34:
                for b in bet:
                    for new in bet:
                        if b!=new:
                            newdp[b*26+new]=dpsum[b]-dp[new*26+b]
            else:
                for key, value in enumerate(dp):
                    v = value % mod2
                    if key//26 != ss and key % 26 != ss:
                        newdp[(key%26)*26+ss] += v
            dp = newdp
        print(sum(dp) % mod2)


if __name__ == '__main__':
    main()