#line 2 "cpplib/util/template.hpp"
#pragma GCC optimize("Ofast")
#pragma GCC optimize("unroll-loops")
#pragma GCC target("avx2")
#include<bits/stdc++.h>
using namespace std;
struct __INIT__{__INIT__(){cin.tie(0);ios::sync_with_stdio(false);cout<<fixed<<setprecision(15);}}__INIT__;
typedef long long lint;
#define INF (1LL<<60)
#define IINF (1<<30)
//#define EPS (1e-10)
#define endl ('\n')
typedef vector<lint> vec;
typedef vector<vector<lint>> mat;
typedef vector<vector<vector<lint>>> mat3;
typedef vector<string> svec;
typedef vector<vector<string>> smat;
template<typename T>using V=vector<T>;
template<typename T>using VV=V<V<T>>;
template<typename T>inline void output(T t){bool f=0;for(auto i:t){cout<<(f?" ":"")<<i;f=1;}cout<<endl;}
template<typename T>inline void output2(T t){for(auto i:t)output(i);}
template<typename T>inline void debug(T t){bool f=0;for(auto i:t){cerr<<(f?" ":"")<<i;f=1;}cerr<<endl;}
template<typename T>inline void debug2(T t){for(auto i:t)debug(i);}
#define loop(n) for(long long _=0;_<(long long)(n);++_)
#define _overload4(_1,_2,_3,_4,name,...) name
#define __rep(i,a) repi(i,0,a,1)
#define _rep(i,a,b) repi(i,a,b,1)
#define repi(i,a,b,c) for(long long i=(long long)(a);i<(long long)(b);i+=c)
#define rep(...) _overload4(__VA_ARGS__,repi,_rep,__rep)(__VA_ARGS__)
#define _overload3_rev(_1,_2,_3,name,...) name
#define _rep_rev(i,a) repi_rev(i,0,a)
#define repi_rev(i,a,b) for(long long i=(long long)(b)-1;i>=(long long)(a);--i)
#define rrep(...) _overload3_rev(__VA_ARGS__,repi_rev,_rep_rev)(__VA_ARGS__)

// #define rep(i,...) for(auto i:range(__VA_ARGS__)) 
// #define rrep(i,...) for(auto i:reversed(range(__VA_ARGS__)))
// #define repi(i,a,b) for(lint i=lint(a);i<(lint)(b);++i)
// #define rrepi(i,a,b) for(lint i=lint(b)-1;i>=lint(a);--i)
// #define irep(i) for(lint i=0;;++i)
// inline vector<long long> range(long long n){if(n<=0)return vector<long long>();vector<long long>v(n);iota(v.begin(),v.end(),0LL);return v;}
// inline vector<long long> range(long long a,long long b){if(b<=a)return vector<long long>();vector<long long>v(b-a);iota(v.begin(),v.end(),a);return v;}
// inline vector<long long> range(long long a,long long b,long long c){if((b-a+c-1)/c<=0)return vector<long long>();vector<long long>v((b-a+c-1)/c);for(int i=0;i<(int)v.size();++i)v[i]=i?v[i-1]+c:a;return v;}
// template<typename T>inline T reversed(T v){reverse(v.begin(),v.end());return v;}
#define all(n) begin(n),end(n)
template<typename T,typename E>bool chmin(T& s,const E& t){bool res=s>t;s=min<T>(s,t);return res;}
template<typename T,typename E>bool chmax(T& s,const E& t){bool res=s<t;s=max<T>(s,t);return res;}
const vector<lint> dx={1,0,-1,0,1,1,-1,-1};
const vector<lint> dy={0,1,0,-1,1,-1,1,-1};
#define SUM(v) accumulate(all(v),0LL)
template<typename T,typename ...Args>auto make_vector(T x,int arg,Args ...args){if constexpr(sizeof...(args)==0)return vector<T>(arg,x);else return vector(arg,make_vector<T>(x,args...));}
#define extrep(v,...) for(auto v:__MAKE_MAT__({__VA_ARGS__}))
#define bit(n,a) ((n>>a)&1)
vector<vector<long long>> __MAKE_MAT__(vector<long long> v){if(v.empty())return vector<vector<long long>>(1,vector<long long>());long long n=v.back();v.pop_back();vector<vector<long long>> ret;vector<vector<long long>> tmp=__MAKE_MAT__(v);for(auto e:tmp)for(long long i=0;i<n;++i){ret.push_back(e);ret.back().push_back(i);}return ret;}
using graph=vector<vector<int>>;
template<typename T>using graph_w=vector<vector<pair<int,T>>>;
template<typename T,typename E>ostream& operator<<(ostream& out,pair<T,E>v){out<<"("<<v.first<<","<<v.second<<")";return out;}
constexpr inline long long powll(long long a,long long b){long long res=1;while(b--)res*=a;return res;}
#line 3 "cpplib/graph_tree/reroot.hpp"

/**
 * @brief 全方位木DP
 */

template<typename T,typename F,typename Fix>
struct reroot{
    std::vector<std::vector<long long>>g;
    std::vector<int>p_list;
    std::vector<T>p_table;
    std::vector<bool>p_checked;
    std::vector<map<int,T>>table;
    std::vector<T>ans;
    T e;
    F f;
    Fix fix;
    reroot(const std::vector<std::vector<long long>>& g,T e,F f=F(),Fix fix=Fix()):g(g),e(e),f(f),fix(fix){
        int n=g.size();
        p_list.resize(n,-1);
        p_checked.resize(n,0);
        table.resize(n);
        p_table.resize(n,e);
        ans.resize(n,e);
        dfs1(0,-1);
        for(int i=0;i<n;++i)ans[i]=dfs2(i,-1);
    }
    T dfs1(int n,int p){
        p_list[n]=p;
        T tmp1=e,tmp2=e;
        std::vector<T>tmp(g[n].size());
        rep(i,g[n].size()){
            int t=g[n][i];
            if(t==p)continue;
            table[n][t]=tmp1;
            tmp1=f(tmp1,tmp[i]=dfs1(t,n));
        }
        for(int i=g[n].size()-1;i>=0;--i){
            int t=g[n][i];
            if(t==p)continue;
            table[n][t]=f(table[n][t],tmp2);
            tmp2=f(tmp[i],tmp2);
        }
        return fix(table[n][p]=tmp1,n,p);
    }
    T dfs2(int n,int p){
        if(n==-1)return e;
        if(!p_checked[n]){
            p_checked[n]=1;
            p_table[n]=dfs2(p_list[n],n);
        }
        if(p==-1){
            return f(table[n][p_list[n]],p_table[n]);
        }else{
            if(p_list[n]==-1)return fix(table[n][p],n,p);
            else return fix(f(table[n][p],p_table[n]),n,p);
        }
    }
    vector<T>query(){
        return ans;
    }
};
#line 3 "code.cpp"

int main(){
    lint n;
    cin>>n;
    string s;
    cin>>s;
    mat v(n);
    map<pair<lint,lint>,char>ma;
    rep(i,n-1){
        lint s,t;
        cin>>s>>t;
        char c;
        cin>>c;
        s--;t--;
        v[s].emplace_back(t);
        v[t].emplace_back(s);
        ma[minmax(s,t)]=c;
    }
    lint m=s.size();
    auto merge=[&](const auto& s,const auto& t){
        vec res(m+1);
        rep(i,m+1)res[i]=max(s[i],t[i]);
        return res;
    };
    auto fix=[&](const auto& dp,lint p,lint q){
        vec res(m+1);
        char c=ma[minmax(p,q)];
        rep(i,m+1){
            chmax(res[i],dp[i]);
            if(i)chmax(res[i],res[i-1]);
            if(i&&s[i-1]==c)chmax(res[i],dp[i-1]+1);
        }
        return res;
    };
    reroot<vec,decltype(merge),decltype(fix)>r(v,vec(m+1),merge,fix);
    auto res=r.query();
    lint ans=0;
    rep(i,n)chmax(ans,res[i].back());
    cout<<ans<<endl;

    // auto dfs=[&](auto dfs,lint now,lint p)->vec{
    //     vec res(m+1);
    //     for(auto [to,c]:v[now]){
    //         if(p==to)continue;
    //         auto dp=dfs(dfs,to,now);
    //         rep(i,m+1){
    //             chmax(res[i],dp[i]);
    //             if(i)chmax(res[i],res[i-1]);
    //             if(i&&s[i-1]==c)chmax(res[i],dp[i-1]+1);
    //         }
    //     }
    //     return res;
    // };
    // lint ans=0;
    // rep(i,n){
    //     chmax(ans,dfs(dfs,i,-1).back());
    // }
    // cout<<ans<<endl;
}