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):
    NDP1=[0]*(26*26)
    NDP2=[0]*(26*26)

    x=S[stind]

    for i in range(26):
        for j in range(26):
            kake=DP[i*26+j]

            if x==-34:
                NDP1[j*26]+=kake
                NDP1[j*26+i]-=kake
                if i!=25:
                    NDP1[j*26+i+1]+=kake
                NDP1[j*26+j]-=kake
                if j!=25:
                    NDP1[j*26+j+1]+=kake
                
            else:
                if i!=x and j!=x:
                    NDP2[j*26+x]+=kake

    for i in range(26):
        for j in range(1,26):
            NDP1[i*26+j]+=NDP1[i*26+j-1]

    DP=[0]*(26*26)

    for i in range(26*26):
        DP[i]=(NDP1[i]+NDP2[i])%mod

print(sum(DP)%mod)