import pypyjit
pypyjit.set_param("max_unroll_recursion=-1")
import sys
sys.setrecursionlimit(10**7)

from functools import cache
MOD = 998244353
N = int(input())
S = input()

@cache
def f(n,p,pp):
    if(n == N):return 1

    ret = 0
    if(S[n] == "?"):
        p_ord = ord(p) - 97
        pp_ord = ord(pp) - 97
        for i in range(26):
            if(p_ord == i or pp_ord == i):continue
            ret += f(n+1, chr(i + 97), p)
            ret %= MOD
    else:
        if(p == S[n] or pp == S[n]):return 0
        ret += f(n+1, S[n], p)
    return ret

print(f(0,"@","@"))