n=int(input())
S=input()
sl=S
sr=S[::-1]
e=[[] for i in range(n)]
for _ in range(n-1):
  a,b,c=input().split()
  a=int(a)-1
  b=int(b)-1
  e[a]+=[(b,c)]
  e[b]+=[(a,c)]
v=[0]*n
ql=[[0]*(len(S)+1) for i in range(n)]
qr=[[0]*(len(S)+1) for i in range(n)]
q=[0]
a=0
while len(q)>0:
  s=q[-1]
  if v[s]==0:
    v[s]=1
    q+=[t for t,c in e[s] if v[t]==0]
  else:
    for t,c in e[s]:
      if v[t]==0:
        nq=[0]*(len(S)+1)
        for i in range(1,len(S)+1):
          nq[i]=max(nq[i-1],ql[t][i])
          if sl[i-1]==c:
            nq[i]=max(nq[i],ql[t][i-1]+1)
        ql[t]=nq[:]
        nq=[0]*(len(S)+1)
        for i in range(1,len(S)+1):
          nq[i]=max(nq[i-1],qr[t][i])
          if sr[i-1]==c:
            nq[i]=max(nq[i],qr[t][i-1]+1)
        qr[t]=nq[:]
    nqr=[[0,0] for i in range(len(S)+1)]
    for t,c in e[s]:
      if v[t]==0:
        for i in range(len(S)+1):
          nqr[i]=sorted(nqr[i]+[qr[t][i]],reverse=True)[:2]
    for t,c in e[s]:
      if v[t]==0:
        a=max(a,ql[t][len(S)],qr[t][len(S)])
        for i in range(len(S)+1):
          a=max(a,ql[t][i]+nqr[len(S)-i][nqr[len(S)-i][0]==qr[t][len(S)-i]])
      for i in range(len(S)+1):
        ql[s][i]=max(ql[s][i],ql[t][i])
        qr[s][i]=max(qr[s][i],qr[t][i])
    v[s]=0
    q.pop()
print(a)