n = int(input())
s = input()
p = 998244353
alptonum = {"?":0,"a":1,"b":2,"c":3,"d":4,"e":5,"f":6,"g":7,"h":8,"i":9,"j":10,"k":11,"l":12,"m":13,"n":14,"o":15,"p":16,"q":17,"r":18,"s":19,"t":20,"u":21,"v":22,"w":23,"x":24,"y":25,"z":26}

if n == 1:
    if s[0] == "?":
        print("{}".format(26))
    else:
        print("{}".format(1))
    exit()

dp = [[[0 for k in range(27)] for i in range(27)] for j in range(n+1)]
if s[0] == "?":
    for i in range(1,27):
        dp[1][0][i] = 1
else:
    dp[1][0][alptonum[s[0]]] = 1

for i in range(1,n):
    if s[i] == "?":
        for j in range(1,27):
            karisum = 0
            for k in range(27):
                karisum = (karisum + dp[i][k][j]) % p
            for k in range(1,27):
                if j != k:
                    dp[i+1][j][k] = (karisum - dp[i][k][j]) % p
    else:
        ind = alptonum[s[i]]
        for j in range(1,27):
            if j != ind:
                for k in range(27):
                    if k != ind:
                        dp[i+1][j][ind] = (dp[i+1][j][ind] + dp[i][k][j]) % p

ans = 0
for i in range(1,27):
    for j in range(1,27):
        ans = (ans + dp[n][i][j]) % p

print("{}".format(ans))