MOD = 998244353
n = int(input())
s = [ord(i)-97 for i in input()]
M = 28
dp = [[0]*M for _ in range(M)]
dp[-1][-1] = 1

def f(v):
    for i in range(M):
        if i == v: continue
        for j in range(M):
            if j == v: continue
            ndp[j][v] += dp[i][j]
            ndp[j][v] %= MOD

for v in s:
    ndp = [[0]*M for _ in range(M)]
    if v == -34:
        for i in range(26):
            f(i)
    else:
        f(v)
    dp = ndp        

print(sum(sum(di) for di in dp)%MOD)