#include <cassert>
#include <cmath>
#include <cstdint>
#include <cstdio>
#include <cstdlib>
#include <cstring>
#include <algorithm>
#include <bitset>
#include <complex>
#include <deque>
#include <functional>
#include <iostream>
#include <limits>
#include <map>
#include <numeric>
#include <queue>
#include <random>
#include <set>
#include <sstream>
#include <string>
#include <unordered_map>
#include <unordered_set>
#include <utility>
#include <vector>

using namespace std;

using Int = long long;

template <class T1, class T2> ostream &operator<<(ostream &os, const pair<T1, T2> &a) { return os << "(" << a.first << ", " << a.second << ")"; };
template <class T> ostream &operator<<(ostream &os, const vector<T> &as) { const int sz = as.size(); os << "["; for (int i = 0; i < sz; ++i) { if (i >= 256) { os << ", ..."; break; } if (i > 0) { os << ", "; } os << as[i]; } return os << "]"; }
template <class T> void pv(T a, T b) { for (T i = a; i != b; ++i) cerr << *i << " "; cerr << endl; }
template <class T> bool chmin(T &t, const T &f) { if (t > f) { t = f; return true; } return false; }
template <class T> bool chmax(T &t, const T &f) { if (t < f) { t = f; return true; } return false; }
#define COLOR(s) ("\x1b[" s "m")


constexpr Int INF = 1001001001001001001LL;

int N;
int WLen;
char W[10];
vector<int> A, B;

string S;

vector<vector<int>> graph;
Int dp[300'010][2][2];

void dfs(int u, int p) {
  for (const int v : graph[u]) if (p != v) {
    dfs(v, u);
  }
  for (int x = 0; x < 2; ++x) for (int y = 0; y < 2; ++y) {
    dp[u][x][y] = -INF;
    if (y == 0) {
      Int sum = 0;
      for (const int v : graph[u]) if (p != v) {
        sum += max(dp[v][y][0], dp[v][y][1]);
      }
      dp[u][x][y] = sum;
    } else {
      Int base = 0;
      vector<Int> diffs;
      for (const int v : graph[u]) if (p != v) {
        base += dp[v][y][0];
        diffs.push_back(dp[v][y][1] - dp[v][y][0]);
      }
      sort(diffs.begin(), diffs.end(), greater<Int>{});
      const Int len = diffs.size();
      Int score = base;
      for (Int k = 0; ; ++k) {
        Int cnt[2] = {len - k, k};
        ++cnt[x];
        Int gain = 0;
        if (false) {
        } else if (S == "BBB") {
          gain = cnt[1] * (cnt[1] - 1);
        } else if (S == "ABB") {
          gain = cnt[0] * cnt[1];
        } else if (S == "ABA") {
          gain = cnt[0] * (cnt[0] - 1);
        } else if (S == "ABC") {
          gain = (cnt[0] / 2) * ((cnt[0] + 1) / 2);
        } else {
          assert(false);
        }
        chmax(dp[u][x][y], score + gain);
        if (k == len) break;
        score += diffs[k];
      }
    }
    assert(dp[u][x][y] >= 0);
  }
// cerr<<u<<": ";for(int x=0;x<2;++x)for(int y=0;y<2;++y)cerr<<dp[u][x][y]<<" ";cerr<<endl;
}

int main() {
  for (; ~scanf("%d", &N); ) {
    scanf("%s", W);
    WLen = strlen(W);
    A.resize(N - 1);
    B.resize(N - 1);
    for (int i = 0; i < N - 1; ++i) {
      scanf("%d%d", &A[i], &B[i]);
      --A[i];
      --B[i];
    }
    
    graph.assign(N, {});
    for (int i = 0; i < N - 1; ++i) {
      graph[A[i]].push_back(B[i]);
      graph[B[i]].push_back(A[i]);
    }
    
    Int ans = -INF;
    if (WLen == 1) {
      ans = N;
    } else if (WLen == 2) {
      ans = N - 1;
    } else if (WLen == 3) {
      if (N < 3) {
        ans = 0;
      } else {
        if (W[0] == W[1] && W[1] == W[2]) {
          S = "BBB";
        } else if (W[0] == W[1] || W[1] == W[2]) {
          S = "ABB";
        } else if (W[0] == W[2]) {
          S = "ABA";
        } else {
          S = "ABC";
        }
cerr<<"S = "<<S<<endl;
        int r = -1;
        for (int u = 0; u < N; ++u) if (graph[u].size() == 1) {
          r = u;
          break;
        }
        assert(~r);
        dfs(r, -1);
        for (int x = 0; x < 2; ++x) for (int y = 0; y < 2; ++y) {
          chmax(ans, dp[graph[r][0]][x][y]);
        }
      }
    } else {
      assert(false);
    }
    printf("%lld\n", ans);
  }
  return 0;
}