N = int(input())
S = list(input())
mod = 998244353

def f(a, b):
    return a * 26 + b

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

pre = [0] * (26 * 26)
Spre = [0] * 26
if S[0] == S[1] == "?":
    for a in range(26):
        for b in range(26):
            if a == b:
                continue
            pre[f(a,b)] = 1
            Spre[b] += 1
elif S[0] == "?":
    b = ord(S[1]) - ord("a")
    for a in range(26):
        if a == b:
            continue
        pre[f(a,b)] = 1
        Spre[b] += 1
elif S[1] == "?":
    a = ord(S[0]) - ord("a")
    for b in range(26):
        if a == b:
            continue
        pre[f(a,b)] = 1
        Spre[b] += 1
else:
    if S[0] == S[1]:
        print(0)
        exit()
    a = ord(S[0]) - ord("a")
    b = ord(S[1]) - ord("a")
    pre[f(a,b)] = 1
    Spre[b] += 1

for i in range(2, N):
    dp = [0] * (26 * 26)
    Sdp = [0] * 26
    if S[i] == "?":
        for a in range(26):
            for b in range(26):
                if a == b:
                    continue
                dp[f(a,b)] += Spre[a] - pre[f(b,a)]
                dp[f(a,b)] %= mod
                Sdp[b] += dp[f(a,b)]
                Sdp[b] %= mod
    else:
        b = ord(S[i]) - ord("a")
        for a in range(26):
            if a == b:
                continue
            dp[f(a,b)] += Spre[a] - pre[f(b,a)]
            dp[f(a,b)] %= mod
            Sdp[b] += dp[f(a,b)]
            Sdp[b] %= mod

    pre, dp = dp, pre
    Spre, Sdp = Sdp, Spre

ans = 0
for a in range(26):
    for b in range(26):
        ans += pre[f(a,b)]
        ans %= mod

print(ans)