import numpy as np en2asc = lambda s: ord(s) - 97 N = int(input()) S = input() MOD = 998244353 dp = np.zeros((27, 27), dtype=np.int64) dp[-1, -1] = 1 for s in S: dp_sum = dp.sum(axis=1) new_dp = np.zeros((27, 27), dtype=np.int64) if s == '?': for i in range(26): tmp = dp_sum - dp[:, i] tmp[i] = 0 new_dp[i] += tmp else: i = en2asc(s) tmp = dp_sum - dp[:, i] tmp[i] = 0 new_dp[i] += tmp dp = new_dp % MOD ans = dp.sum() % MOD print(ans)