#include<bits/stdc++.h>
using namespace std; const int N=100010; using ll=long long; char s[N]; vector<int> e[N]; ll c[N],w[N],wc[N],ww[N],r;
void dfs(int u,int f){
    for(int v:e[u]){
        if(v==f) continue; dfs(v,u);
        c[u]+=c[v],w[u]+=w[v],wc[u]+=wc[v],ww[u]+=ww[v],r-=w[v]*wc[v]+ww[v]*c[v];
    }
    r+=w[u]*wc[u]+ww[u]*c[u];
    if(s[u]=='c') c[u]++,r+=ww[u]; else r+=wc[u],wc[u]+=c[u],ww[u]+=w[u],w[u]++;
}
int main(){
    int n; scanf("%d%s",&n,s+1);
    for(int i=1,u,v;i<n;++i) scanf("%d%d",&u,&v),e[u].push_back(v),e[v].push_back(u);
    dfs(1,0); printf("%lld\n",r); return 0;
}