import sys
N = int(input())
S = input()
mod = 998244353
dp = [[[0] * 26 for i in range(26)] for j in range(N)]

def code(s):
    return ord(s) - 97

if N == 1:
    if S[0] == "?":
        print(26)
        sys.exit()
    else:
        print(1)
        sys.exit()

if S[0:2] == "??":
    for i in range(26):
        for j in range(26):
            if i != j:
                dp[1][i][j] = 1

elif S[0] == "?":
    j = code(S[1])
    for i in range(26):
        if i != j:
            dp[1][i][j] = 1

elif S[1] == "?":
    j = code(S[0])
    for i in range(26):
        if i != j:
            dp[1][j][i] = 1

else:
    if S[0] != S[1]: 
        dp[1][code(S[0])][code(S[1])] = 1

for i in range(2,N):
    if S[i] == "?":
        AS = [[0] * 27 for _ in range(26)]
        for k in range(26):
            for j in range(26):
                AS[k][j+1] = AS[k][j] + dp[i-1][j][k]
        for k in range(26):
            for l in range(26):
                if k != l:
                    temp = AS[k][l] + AS[k][-1] - AS[k][l+1]
                    dp[i][k][l] = temp % mod

    else:
        x = code(S[i])
        for j in range(26):
            for k in range(26):
                if j != x and k != x:
                    dp[i][k][x] = (dp[i][k][x] + dp[i-1][j][k]) % mod

ans = 0
for i in range(26):
    ans = (ans + sum(dp[-1][i])) % mod
print(ans)