def main():
    N = int(input()); MOD = 998244353
    S = str(input())
    if N == 1:
        if S == "?":
            print(26)
        else:
            print(1)
        exit()

    MAX = 26*26
    #dp[i]: 一つ前がi//26,二つ前がi%26の場合    
    dp = [0]*MAX
    dp_pre = [0]*26
    #dp_prepre = [0]*26
    for i in range(26): #二つ前
        for j in range(26):
            if i == j: continue
            if S[0] == chr(i+97) or S[0] == "?":
                if S[1] == chr(j+97) or S[1] =="?":
                    idx = j*26 + i
                    dp[idx] = 1
    for i in range(26):
        for j in range(26):
            idx = j*26 + i
            dp_pre[j] += dp[idx]
            #dp_prepre[i] += dp[idx]
    #print(dp)
    #print(dp_pre)
    #print(dp_prepre)
    for i in range(2,N):
        p = [0]*MAX
        p_pre = [0]*26
        #p_prepre = [0]*26
        p,dp = dp,p
        dp_pre,p_pre = p_pre,dp_pre
        #dp_prepre,p_prepre = p_prepre,dp_prepre
        #SUM = sum(p)
        if S[i] != "?":
            si = ord(S[i]) - 97
            for pre in range(26):
                if si == pre: continue #一つ前と同じではだめ
                nidx = si*26 + pre
                dp[nidx] += p_pre[pre] - p[pre*26 + si]
                #print(dp[nidx])
                dp[nidx] %= MOD
        else: #?
            for si in range(26): #今
                for pre in range(26): #今の前
                    if si == pre: continue
                    nidx = si*26 + pre
                    dp[nidx] += p_pre[pre] - p[pre*26 + si]
                    #print(dp[nidx])
                    dp[nidx] %= MOD
        
        for j in range(26): #二つ前
            for k in range(26): #一つ前
                if j == k: continue
                jkidx = k*26 + j
                dp_pre[k] += dp[jkidx]
                #dp_prepre[j] += dp[jkidx]

        #print(dp)
        #print(dp_pre)
    
    ans = sum(dp)%MOD
    print(ans)


if __name__ == '__main__':
    main()