"""

dp[i+1][yz] = sum(dp[i][?y]) - dp[i][zy] 

"""


import itertools

N = int(input())
S = input()

dp = [0] * (27**2)
dp[-1] = 1
mod = 998244353

for i in range(N):

    dps = [0] * 27
    for j in range(27**2):
        dps[j % 27] += dp[j]
    
    ndp = [0] * (27**2)

    if S[i] == "?":
        for yz in range(27**2):
            y = yz // 27
            z = yz % 27
            if z == 26 or y == z:
                continue
            ndp[yz] = dps[y] - dp[z*27+y]
            ndp[yz] %= mod
    else:
        z = ord(S[i])-ord('a')
        for y in range(27):
            if y == z:
                continue
            ndp[27*y+z] = dps[y] - dp[z*27+y]
            ndp[27*y+z] %= mod

    dp = ndp
    
print (sum(dp) % mod)