N=int(input())
S=list(input())
for i in range(N):
  if S[i]=='?':
    S[i]=-1
  else:
    S[i]=ord(S[i])-ord('a')
DP=[[[0]*27 for j in range(27)] for i in range(N+1)]
DP[0][26][26]=1
mod=998244353
for i in range(N):
  cx=[0]*27
  cy=[0]*27
  for j in range(27):
    for k in range(27):
      cx[j]=(cx[j]+DP[i][j][k])%mod
      cy[k]=(cy[k]+DP[i][j][k])%mod
  a=S[i]
  for j in range(26):
    for k in range(27):
      if a==j:
        if j!=k:
          DP[i+1][j][k]=(cx[k]-DP[i][k][a])%mod
      if a==-1:
        if j!=k:
          DP[i+1][j][k]=(cx[k]-DP[i][k][j])%mod
print(sum([sum(DP[N][i]) for i in range(27)])%mod)