#include <string>
#include <vector>
#include<iostream>
#include<cstdio>
#include<cstdlib>
#include<stack>
#include<queue>
#include<cmath>
#include<algorithm>
#include<functional>
#include<list>
#include<deque>
#include<bitset>
#include<set>
#include<map>
#include<unordered_map>
#include<unordered_set>
#include<cstring>
#include<sstream>
#include<complex>
#include<iomanip>
#include<numeric>
#include<cassert>
#include<random>
#define X first
#define Y second
#define pb push_back
#define rep(X,Y) for (int (X) = 0;(X) < (int)(Y);++(X))
#define reps(X,S,Y) for (int (X) = (int)(S);(X) < (int)(Y);++(X))
#define rrep(X,Y) for (int (X) = (int)(Y)-1;(X) >=0;--(X))
#define rreps(X,S,Y) for (int (X) = (int)(Y)-1;(X) >= (int)(S);--(X))
#define repe(X,Y) for ((X) = 0;(X) < (Y);++(X))
#define peat(X,Y) for (;(X) < (Y);++(X))
#define all(X) (X).begin(),(X).end()
#define rall(X) (X).rbegin(),(X).rend()
#define eb emplace_back
#define UNIQUE(X) (X).erase(unique(all(X)),(X).end())
#define Endl endl
#define NL <<"\n"
#define cauto const auto

using namespace std;
using ll=long long;
using pii=pair<int,int>;
using pll=pair<ll,ll>;
template<class T> using vv=vector<vector<T>>;
template<class T> inline bool MX(T &l,const T &r){return l<r?l=r,1:0;}
template<class T> inline bool MN(T &l,const T &r){return l>r?l=r,1:0;}
//#undef NUIP
#ifdef NUIP
#include "benri.h"
#else
#define out(args...)
#endif
void ouT(ll x,int d=3){auto re=to_string(x);if((int)re.size()>d) re=x>0?"oo":"-oo";cout<<string(d-re.size(),' ')<<re<<",";}
#ifdef __cpp_init_captures
template<typename T>vector<T> table(int n, T v){ return vector<T>(n, v);}
template <class... Args> auto table(int n, Args... args){auto val = table(args...); return vector<decltype(val)>(n, move(val));}
#endif
template<class A,class B> pair<A,B> operator+(const pair<A,B> &p,const pair<A,B> &q){ return {p.X+q.X,p.Y+q.Y};}
template<class A,class B,class C,class D> pair<A,B>& operator+=(pair<A,B> &p,const pair<C,D> &q){ p.X+=q.X; p.Y+=q.Y; return p;}
template<class A,class B> pair<A,B> operator-(const pair<A,B> &p,const pair<A,B> &q){ return {p.X-q.X,p.Y-q.Y};}
template<class A,class B,class C,class D> pair<A,B>& operator-=(pair<A,B> &p,const pair<C,D> &q){ p.X-=q.X; p.Y-=q.Y; return p;}
template<class A,class B> istream& operator>>(istream &is, pair<A,B> &p){ is>>p.X>>p.Y; return is;}
template<class T=ll> T read(){ T re; cin>>re; return move(re);}
template<class T=ll> T read(const T &dec){ T re; cin>>re; return re-dec;}
template<class T=ll> vector<T> readV(const int sz){ vector<T> re(sz); for(auto &x:re) x=read<T>(); return move(re);}
template<class T=ll> vector<T> readV(const int sz, const T &dec){ vector<T> re(sz); for(auto &x:re) x=read<T>(dec); return move(re);}
vv<int> readG(const int &n,const int &m){ vv<int> g(n); rep(_,m){ cauto a=read<int>(1),b=read<int>(1); g[a].pb(b); g[b].pb(a);} return move(g);}
vv<int> readG(const int &n){ return readG(n,n-1);}
#define TT cauto TTT=read();rep(_,TTT)
const ll MOD=1e9+7; //998244353;

#include <ext/pb_ds/assoc_container.hpp>
using namespace __gnu_pbds;


using Map=gp_hash_table<int, int>;

int main(){
  ios_base::sync_with_stdio(false); cin.tie(0);
  cout<<fixed<<setprecision(0);
	cauto n=read();
	cauto s=read<string>();
	vv<pair<int,char>> g(n);
	rep(_,n-1){
		cauto u=read(1);
		cauto v=read(1);
		cauto c=read<char>();
		g[u].eb(v,c);
		g[v].eb(u,c);
	}
	vector<Map> dp(n);
	cauto dfs1=
		[&](auto &&dfs,int v,int p)->void{
			for(cauto &[u,ignore]:g[v])if(u!=p){
					dfs(dfs,u,v);
					if(p>=0) MX(dp[p][v],dp[v][u]);
				}
		};
	cauto dfs2=
		[&](auto &&dfs,int v,int p)->void{
			pii fst(dp[v][p],p),snd(0,-1);
			for(cauto &[u,val]:dp[v]){
				const pii p(val,u);
				if(fst<=p) snd=fst, fst=p;
				else if(snd<=p) snd=p;
			}
			for(cauto &[u,ignore]:g[v])if(u!=p){
					cauto &pr=(fst.Y==u?snd:fst);
					MX(dp[u][v],pr.X);
					dfs(dfs,u,v);
				}
		};
	for(cauto &c:s){
		vector<Map> nxt(n);
		rep(v,n){
			pii fst(0,-1),snd(0,-1);
			for(cauto &[u,ignore]:g[v]){
				const pii p(dp[v][u],u);
				if(fst<=p) snd=fst, fst=p;
				else if(snd<=p) snd=p;
			}
			for(cauto &[u,d]:g[v]){
				cauto &p=(fst.Y==u?snd:fst);
				MX(nxt[u][v],p.X+(c==d));
			}
			dfs1(dfs1,0,-1);
			dfs2(dfs2,0,-1);
		}
		rep(v,n)for(cauto &[u,val]:nxt[v]) MX(dp[v][u],val);
	}
	// out(dp,1);
	int re=0;
	rep(v,n)for(cauto &[u,val]:dp[v]) MX(re,val);
	cout<<re NL;
  return 0;
}