#include<bits/stdc++.h>
namespace {
#pragma GCC diagnostic push
#pragma GCC diagnostic ignored "-Wunused-function"
#include<atcoder/all>
#pragma GCC diagnostic pop
using namespace std;
using namespace atcoder;
#define rep(i,n) for(int i = 0; i < (int)(n); i++)
#define rrep(i,n) for(int i = (int)(n) - 1; i >= 0; i--)
#define all(x) begin(x), end(x)
#define rall(x) rbegin(x), rend(x)
template<class T> bool chmax(T& a, const T& b) { if (a < b) { a = b; return true; } else return false; }
template<class T> bool chmin(T& a, const T& b) { if (b < a) { a = b; return true; } else return false; }
using ll = long long;
using P = pair<int,int>;
using VI = vector<int>;
using VVI = vector<VI>;
using VL = vector<ll>;
using VVL = vector<VL>;

constexpr int A = 'A' - 'A';
constexpr int G = 'G' - 'A';
constexpr int C = 'C' - 'A';
constexpr int T = 'T' - 'A';

} int main() {
  ios::sync_with_stdio(false);
  cin.tie(0);
  int n;
  cin >> n;
  VI st1(n), st2;
  int cnt[26]{};  // in original
  int rot = 0;
  rep(i, n) {
    char c;
    cin >> c;
    int x = c - 'A';
    st1[i] = x;
    cnt[x]++;
  }
  int ans = 0;
  auto is_AGCT = [](int x) {
    return x == A || x == G || x == C || x == T;
  };
  while(true) {
    int c1 = 0;
    rep(i, 26) if (is_AGCT((i + rot) % 26)) c1 += cnt[i];
    if (c1 == 0) break;
    ans++;
    while((int)st1.size() > c1) st2.emplace_back(st1.back()), st1.pop_back();
    while((int)st1.size() < c1) st1.emplace_back(st2.back()), st2.pop_back();
    int x = st1.back();
    st1.pop_back();
    int c2 = --cnt[x];
    rot += c2;
  }
  cout << ans << '\n';
}