n = int(input())
S = input()
mod = 998244353
dp = [0]*(26**2)
if len(S) == 1:
    if S == "?":
        ans = 26
    else:
        ans = 1
    print(ans)
    exit()

if S[0] == "?":
    if S[1] == "?":
        for i in range(26):
            for j in range(26):
                if i == j:
                    continue
                dp[i*26+j] += 1
    else:
        c = ord(S[1])-ord("a")
        for i in range(26):
            if i == c:
                continue
            dp[i*26+c] += 1
else:
    c = ord(S[0])-ord("a")
    if S[1] == "?":
        for j in range(26):
            if c == j:
                continue
            dp[c*26+j] += 1
    else:
        c2 = ord(S[1])-ord("a")
        if c != c2:
            dp[c*26+c2] += 1
base = [0]*26
for s in S[2:]:
    nbase = [0]*26
    ndp = [0]*(26**2)
    if s == "?":
        for i in range(26**2):
            x,y = divmod(i,26)
            dp[i] += base[x]
            nbase[y] += dp[i]
            nbase[y] %= mod
            ndp[y*26+y] -= dp[i]
            ndp[y*26+y] %= mod
            ndp[y*26+x] -= dp[i]
            ndp[y*26+x] %= mod

    else:
        c = ord(s)-ord("a")
        for i in range(26**2):
            x,y = divmod(i,26)
            dp[i] += base[x]
            if x == c or y == c:
                continue
            ndp[y*26+c] += dp[i]
            ndp[y*26+c] %= mod
    dp = ndp
    base = nbase
ans = 0
for i in range(26):
    for j in range(26):
        ans += dp[i*26+j]+base[i]
        ans %= mod
print(ans)