import sys
input = sys.stdin.readline
N = int(input())
S = list(input())[: -1]
mod = 998244353

dp = [[0] * 26 for _ in range(26)]

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

for x in range(26):
  for y in range(26):
    if x == y: continue
    cx = chr(ord("a") + x)
    cy = chr(ord("a") + y)

    if S[0] == cx and S[1] == cy: dp[x][y] += 1
    if S[0] == "?" and S[1] == cy: dp[x][y] += 1
    if S[0] == cx and S[1] == "?": dp[x][y] += 1
    if S[0] == "?" and S[1] == "?": dp[x][y] += 1
#print(dp)
for c in S[2: ]:
  z = ord(c) - ord("a")
  dp2 = [[0] * 26 for _ in range(26)]
  if c != "?":
    for x in range(26):
      for y in range(26):
        cx = chr(ord("a") + x)
        cy = chr(ord("a") + y)
        if x != z and y != z:
          dp2[y][z] += dp[x][y]
          dp2[y][z] %= mod
  else:
    sm = [0] * 26
    for x in range(26):
      for y in range(26):
        cx = chr(ord("a") + x)
        cy = chr(ord("a") + y)
        dp2[y][y] -= dp[x][y]
        dp2[y][y] %= mod
        dp2[y][x] -= dp[x][y]
        dp2[y][y] %= mod

        sm[y] += dp[x][y]
        sm[y] %= mod

    for x in range(26):
      for y in range(26):
        dp2[x][y] += sm[x]
        dp2[x][y] %= mod

  dp = [t[: ] for t in dp2]

res = 0
for x in range(26):
  for y in range(26):
    res += dp[x][y]
    res %= mod
print(res)