#include <bits/stdc++.h>

#define _overload(_1,_2,_3,name,...) name
#define _rep(i,n) _range(i,0,n)
#define _range(i,a,b) for(int i=int(a);i<int(b);++i)
#define rep(...) _overload(__VA_ARGS__,_range,_rep,)(__VA_ARGS__)

#define _rrep(i,n) _rrange(i,n,0)
#define _rrange(i,a,b) for(int i=int(a)-1;i>=int(b);--i)
#define rrep(...) _overload(__VA_ARGS__,_rrange,_rrep,)(__VA_ARGS__)

#define _all(arg) begin(arg),end(arg)
#define uniq(arg) sort(_all(arg)),(arg).erase(unique(_all(arg)),end(arg))
#define getidx(ary,key) lower_bound(_all(ary),key)-begin(ary)
#define clr(a,b) memset((a),(b),sizeof(a))
#define bit(n) (1LL<<(n))
#define popcount(n) (__builtin_popcountll(n))

using namespace std;

template<class T>bool chmax(T &a, const T &b) { return (a<b)?(a=b,1):0;}
template<class T>bool chmin(T &a, const T &b) { return (b<a)?(a=b,1):0;}

using ll=long long;
using R=long double;
const R EPS=1e-9; // [-1000,1000]->EPS=1e-8 [-10000,10000]->EPS=1e-7
inline int sgn(const R& r){return(r > EPS)-(r < -EPS);}
inline R sq(R x){return sqrt(max<R>(x,0.0));}

const int dx[8]={1,0,-1,0,1,-1,-1,1};
const int dy[8]={0,1,0,-1,1,1,-1,-1};

// Problem Specific Parameter:

const int nmax=100010;
string s;
vector<int> graph[nmax];
unordered_map<int,ll> dpc[nmax];
unordered_map<int,ll> dpw[nmax];


void dfs(int v,int p){
	if(dpc[v].find(p)!=end(dpc[v])) return;
	dpc[v][p]=(s[v]=='c');
	dpw[v][p]=(s[v]=='w');	
	for(auto &v2:graph[v]){
		if(v2==p) continue;
		dfs(v2,v);
		dpc[v][p]+=dpc[v2][v];
		dpw[v][p]+=dpw[v2][v];
	}
}

int main(void){
	int n;
	cin >> n >> s;
	
	rep(i,n-1){
		int a,b;
		cin >> a >> b;
		graph[a-1].push_back(b-1);
		graph[b-1].push_back(a-1);
	}

	rep(v,n) for(auto &v2:graph[v]) dfs(v,v2);

	ll ans=0LL;

	rep(v,n){
		if(s[v]=='c') continue;
		ll tc=0LL,tw=0LL;
		for(auto &v2:graph[v]){
			if(v2==-1) continue;
			tc+=dpc[v2][v];
			tw+=dpw[v2][v];
			ans-=1LL*dpc[v2][v]*dpw[v2][v];
		}
		ans+=1LL*tc*tw;
	}

	cout << ans << endl;
	return 0;
}