import sys
input = sys.stdin.readline

mod = 998244353
N = int(input())
S = input().rstrip()
if N == 1:
    if S[0] == '?':
        print(26)
    else:
        print(1)
    exit()
dp = [0] * (26**2)
for a in range(26):
    if S[0] != '?' and S[0] != chr(ord('a') + a):
        continue
    for b in range(26):
        if S[1] != '?' and S[1] != chr(ord('a') + b):
            continue
        if a != b:
            dp[26*a + b] = 1
for i in range(2, N):
    ndp = [0] * (26**2)
    for j in range(26):
        if S[i] != '?' and S[i] != chr(ord('a') + j):
            continue
        for k in range(26**2):
            a, b = divmod(k, 26)
            if a != j and b != j:
                ndp[26*b + j] = (ndp[26*b + j] + dp[k]) % mod
    dp = ndp
ans = 0
for i in range(26**2):
    ans = (ans + dp[i]) % mod
print(ans)