from string import ascii_lowercase as X
from collections import defaultdict
N=int(input())
S=["*"]
for s in input():
    if s=="?":
        S.append(-1)
    else:
        S.append(ord(s)-ord("a"))
Mod=998244353

#==================================================
def answer(x):
    exit(print(x))

def f(a,b):
    return 26*a+b

#==================================================
if N==1:
    if S[1]==-1:
        answer(26)
    else:
        answer(1)

for i in range(1,N):
    if S[i]==S[i+1] and S[i]!=-1:
        answer(0)

for i in range(1,N-1):
    if S[i]==S[i+2] and S[i]!=-1:
        answer(0)

D=[0]*676
R=range(26)
if S[1]==-1:
    if S[2]==-1:
        for a in R:
            for b in R:
                if a!=b:
                    D[f(a,b)]=1
    else:
        for a in R:
            if a!=S[2]:
                D[f(a,S[2])]=1
else:
    if S[2]==-1:
        for b in R:
            if S[1]!=b:
                D[f(S[1],b)]=1
    else:
        D[f(S[1],S[2])]=1

for c in S[3:]:
    E=D
    D=[0]*676

    if c==-1:
        L=[0]*26
        M=[0]*676

        for i in range(676):
            a,b=divmod(i,26)
            L[b]+=E[f(a,b)]
            M[f(b,a)]+=E[f(a,b)]
            M[f(b,b)]+=E[f(a,b)]

        for a in R:
            for b in R:
                D[f(a,b)]=L[a]-M[f(a,b)]
    else:
        for i in range(676):
            a,b=divmod(i,26)
            if a!=c and b!=c:
                D[f(b,c)]+=E[f(a,b)]

    for i in range(676):
        D[i]%=Mod

print(sum(D)%Mod)