import sys
input = sys.stdin.readline
from collections import *

N = int(input())
S = input()[:-1]

if N==1:
    if S=='?':
        print(26)
    else:
        print(1)
    exit()
    
dp = [[0]*26 for _ in range(26)]
MOD = 998244353

for i in range(26):
    for j in range(26):
        if i!=j and (S[1]=='?' or ord(S[1])-ord('a')==i) and (S[0]=='?' or ord(S[0])-ord('a')==j):
            dp[i][j] = 1

for i in range(2, N):
    sus = []
    
    for j in range(26):
        su = 0
        
        for k in range(26):
            su += dp[j][k]
            su %= MOD
        
        sus.append(su)
        
    ndp = [[0]*26 for _ in range(26)]
    
    for j in range(26):
        if S[i]=='?' or ord(S[i])-ord('a')==j:
            for k in range(26):
                if j!=k:
                    ndp[j][k] = (sus[k]-dp[k][j])%MOD
    
    dp = ndp

ans = 0

for i in range(26):
    for j in range(26):
        ans += dp[i][j]
        ans %= MOD

print(ans)