N = int(input())
S = input()
if N==1:
    print(26 if S[0]=='?' else 1)
    exit()

def f(s):
    return ord(s)-97
mod = 998244353
dp = [[0]*26 for _ in range(26)]
if S[0]=='?' and S[1]=='?':
    for i in range(26):
        for j in range(26):
            if i==j:continue
            dp[i][j] = 1
elif S[0]=='?':
    s = f(S[1])
    for i in range(26):
        if i==s:continue
        dp[i][s] = 1
elif S[1]=='?':
    s = f(S[0])
    for i in range(26):
        if i==s:continue
        dp[s][i] = 1
else:
    s1, s2 = f(S[0]), f(S[1])
    dp[s1][s2] = 1

for i in range(2,N):
    ndp = [[0]*26 for _ in range(26)]
    sm = [0]*26
    for j in range(26):
        for k in range(26):
            if j==k:continue
            sm[k] += dp[j][k]
            sm[k] %= mod
    if S[i]=='?':
        for j in range(26):
            for k in range(26):
                if j==k:continue
                ndp[j][k] += sm[j]-dp[k][j]
                ndp[j][k] %= mod
    else:
        s = f(S[i])
        for j in range(26):
            ndp[j][s] += sm[j]-dp[s][j]
            ndp[j][s] %= mod
    dp = ndp
res = 0
for i in range(26):
    for j in range(26):
        if i==j:continue
        res += dp[i][j]
        res %= mod
print(res)