n=int(input())
s=input()
M=998244353
q=[[0]*27 for i in range(27)]
q[-1][-1]=1
for c in s:
  nq=[[0]*27 for i in range(27)]
  for i1 in range(27):
    y=sum(q[i1])%M
    for i3 in range(26):
      if i3!=i1:
        if c==chr(i3+ord("a")) or c=="?":
          nq[i3][i1]=(y-q[i1][i3])%M
  q=nq
print(sum(sum(q[i]) for i in range(27))%M)