import sys
input = lambda : sys.stdin.readline().rstrip()

sys.setrecursionlimit(2*10**5+10)
write = lambda x: sys.stdout.write(x+"\n")
debug = lambda x: sys.stderr.write(x+"\n")

# 木の読み込み・重みあり
n = int(input())
s = [(ord(c)-ord("a")) for c in input()]
m = len(s)
ns = [[] for _ in range(n)]
for _ in range(n-1):
    u,v,c = input().split()
    c = ord(c) - ord("a")
    u = int(u)
    v = int(v)
    u -= 1
    v -= 1
    ns[u].append((c,v))
    ns[v].append((c,u))
dp0 = [-1]*n
dp1 = [-1]*n
ans = 0
def sub(l,c):
    nl = [0]
    for i in range(m):
        if s[i]==c:
            v = l[i]+1
        else:
            v = l[i]
        nl.append(max(v, nl[-1]))
    return nl
def sub2(l,c):
    nl = [0]
    for i in range(m)[::-1]:
        if s[i]==c:
            v = l[i+1]+1
        else:
            v = l[i]
        nl.append(max(v, nl[-1]))
    return nl[::-1]
def sub3(l0,l1):
    k = len(l0)
    ans = 0
    for i in range(k):
        for j in range(k):
            if i==j:
                continue
            ans = max(ans, max((l0[i][ind]+l1[j][ind] for ind in range(len(l0)))))
    ans = max(ans, max((item[-1] for item in l0)), max((item[0] for item in l1)))
    return ans
# 深さ優先探索 (巻き戻しあり) dfs tree
q = [(0, -1)]
while q:
    u,prv = q.pop()
    if u<0:
        # 返るときの処理
        u = ~u
        l0 = []
        l1 = []
        for c,v in ns[u]:
            if v==prv:
                continue
            v0 = sub(dp0[v],c)
            v1 = sub2(dp1[v],c)
            l0.append(v0)
            l1.append(v1)
#             print(c,v0,v1)
        if l0:
            ans = max(ans, sub3(l0,l1))
            dp0[u] = [max(item[i] for item in l0) for i in range(m+1)]
            dp1[u] = [max(item[i] for item in l1) for i in range(m+1)]
        else:
            dp0[u] = [0]*(m+1)
            dp1[u] = [0]*(m+1)
    else:
        q.append((~u,prv))
        for c,v in ns[u]:
            # 進むときの処理
            if v==prv:
                continue
            q.append((v,u))
print(ans)