#include <bits/stdc++.h>
using namespace std;

int dp[2005][2005];
vector<int>tmp[30];
vector<pair<int,char>>ki[2005];

void dfs(int n,int p) {
    char c;
    for(int i = 0; i < ki[n].size(); i++) {
        if(ki[n][i].first == p) {
            c = ki[n][i].second;
            continue;
        }
        dfs(ki[n][i].first,n);
    }
    if(p == -1) {
        return;
    }
    for(int i = 0; i <= 2000; i++) {
        dp[p][i] = max(dp[p][i],dp[n][i]);
        int it = lower_bound(tmp[c-'a'].begin(),tmp[c-'a'].end(),i)-tmp[c-'a'].begin();
        if(it != tmp[c-'a'].size()) {
            dp[p][tmp[c-'a'][it]+1] = max(dp[p][tmp[c-'a'][it]+1],dp[n][i]+1);
        }
    }
}

void dfs2(int n,int p) {
    vector<vector<int>>mx(ki[n].size(),vector<int>(2005));
    for(int i = 0; i < ki[n].size(); i++) {
        for(int j = 0; j <= 2000; j++) {
            mx[i][j] = max(mx[i][j],dp[ki[n][i].first][j]);
            char c = ki[n][i].second;
            int it = lower_bound(tmp[c-'a'].begin(),tmp[c-'a'].end(),j)-tmp[c-'a'].begin();
            if(it != tmp[c-'a'].size()) {
                mx[i][tmp[c-'a'][it]+1] = max(mx[i][tmp[c-'a'][it]+1],dp[ki[n][i].first][j]+1);
            }
        }
    }
    vector<vector<int>>l(ki[n].size(),vector<int>(2005)),r(ki[n].size(),vector<int>(2005));
    for(int i = 0; i < ki[n].size(); i++) {
        for(int j = 0; j <= 2000; j++) {
            l[i][j] = mx[i][j];
            r[i][j] = mx[i][j];
        }
    }
    for(int i = 0; i < ki[n].size()-1; i++) {
        for(int j = 0; j <= 2000; j++) {
            l[i+1][j] = max(l[i+1][j],l[i][j]);
        }
    }
    for(int i = ki[n].size()-1; i >= 1; i--) {
        for(int j = 0; j <= 2000; j++) {
            r[i-1][j] = max(r[i-1][j],r[i][j]);
        }
    }
    for(int i = 0; i < ki[n].size(); i++) {
        if(ki[n][i].first == p) {
            continue;
        }
        for(int j = 0; j <= 2000; j++) {
            int cnt = 0;
            if(i) {
                cnt = max(cnt,l[i-1][j]);
            }
            if(i+1 < ki[n].size()) {
                cnt = max(cnt,r[i+1][j]);
            }
            dp[n][j] = cnt;
        }
        dfs2(ki[n][i].first,n);
    }
    for(int i = 0; i <= 2000; i++) {
        dp[n][i] = l[ki[n].size()-1][i];
    }
}

int main() {
    int N;
    string S;
    cin >> N >> S;
    for(int i = 0; i < N-1; i++) {
        int u,v;
        char c;
        cin >> u >> v >> c;
        u--;
        v--;
        ki[u].push_back({v,c});
        ki[v].push_back({u,c});
    }
    for(int i = 0; i < S.size(); i++) {
        tmp[S[i]-'a'].push_back(i);
    }
    dfs(0,-1);
    int ans = 0;
    for(int i = 0; i < N; i++) {
        for(int j = 0; j <= S.size(); j++) {
            ans = max(ans,dp[i][j]);
        }
    }
    dfs2(0,-1);
    for(int i = 0; i < N; i++) {
        for(int j = 0; j <= S.size(); j++) {
            ans = max(ans,dp[i][j]);
        }
    }
    cout << ans << endl;
}