import sys
input = sys.stdin.readline

mod=998244353

N=int(input())
S=list(input().strip())

for i in range(N):
    S[i]=ord(S[i])-97

if N==1:
    print(1)
    exit()

DP=[0]*(26*26)

for i in range(26):
    for j in range(26):
        if i==j:
            continue

        if (i==S[0] or -34==S[0]) and (j==S[1] or -34==S[1]):
            DP[i*26+j]=1

#print(DP)

for stind in range(2,N):
    NDP=[0]*(26*26)

    x=S[stind]

    for i in range(26):
        for j in range(26):
            kake=DP[i*26+j]
            
            for k in range(26):
                if (x==k or x==-34) and i!=k and j!=k:
                    NDP[j*26+k]+=kake
                    NDP[j*26+k]%=mod

    DP=NDP

print(sum(DP)%mod)