#include<bits/stdc++.h>
using std::cin;
using std::cout;
using std::endl;
using ll = long long;
using std::vector;
using std::string;
#define rep(i, j, n) for(int i = int(j); i < int(n); i++)
#define REP(i, j, n) for(int i = int(j); i <= int(n); i++)
#define per(i, j, n) for(int i = int(j); int(n) <= i; i--)
ll N, M;
string S;
vector<vector<ll>> edge, dp, rev;
vector<vector<char>> c;
vector<bool> flag;
ll ans = 0;

void dfs(ll now){
    flag[now] = true;
    rep(i,0,edge[now].size()){
        ll next = edge[now][i];
        ll ch = c[now][i];
        if(flag[next]) continue;
        dfs(next);
        per(j,M,1){
            dp[next][j] = std::max(dp[next][j], dp[next][j-1]+(S[j-1]==ch));
        }
        REP(j,1,M){
            dp[next][j] = std::max(dp[next][j], dp[next][j-1]);
        }
        REP(j,1,M){
            rev[next][j-1] = std::max(rev[next][j-1], rev[next][j]+(S[j-1]==ch));
        }
        per(j,M,1){
            rev[next][j-1] = std::max(rev[next][j-1], rev[next][j]);
        }
        REP(j,0,M){
            ans = std::max(ans, dp[now][j]+rev[next][j]);
            ans = std::max(ans, dp[next][j]+rev[now][j]);
        }
        REP(j,0,M){
            dp[now][j] = std::max(dp[now][j], dp[next][j]);
            rev[now][j] = std::max(rev[now][j], rev[next][j]);
        }
    }
}

int main(){
    ll max = 2000;
    cin >> N >> S;
    assert(N<=max && 2<=N && 1<=S.size() && S.size()<=max);
    rep(i,0,S.size()){
        assert('a'<=S[i] && S[i]<='z');
    }
    M = S.size();
    edge.resize(N);
    c.resize(N);
    rep(i,1,N){
        ll u, v;
        char c_;
        cin >> u >> v >> c_;
        assert(1<=u && u<v && v<=N);
        assert('a'<=c_ && c_<='z');
        edge[--u].push_back(--v);
        edge[v].push_back(u);
        c[u].push_back(c_);
        c[v].push_back(c_);
    }
    dp.resize(N, vector<ll>(M+1));
    rev.resize(N, vector<ll>(M+1));
    flag.resize(N);
    dfs(0);
    cout << ans << endl;
    rep(i,0,N) assert(flag[i]);
    assert(ans == 2);
}