#include <bits/stdc++.h>

#include <atcoder/mincostflow>
#include <atcoder/modint>

using namespace atcoder;
using namespace std;
#define REP(i, m, n) for (int64_t i = (int64_t)(m); i < (int64_t)(n); i++)
#define rep(i, n) REP(i, 0, n)
#define RREP(i, m, n) for (int64_t i = (int64_t)(m - 1); i >= (int64_t)(n); i--)
#define rrep(i, n) RREP(i, n, 0)
#define all(v) v.begin(), v.end()
using vi = vector<int64_t>;
using vvi = vector<vi>;
using vb = vector<bool>;
using vvb = vector<vb>;
using vc = vector<char>;
using vvc = vector<vc>;
using vs = vector<string>;
using vvs = vector<vs>;
using vd = vector<long double>;
using vvd = vector<vd>;
using pii = pair<int64_t, int64_t>;
using tii = tuple<int64_t, int64_t, int64_t>;
using vp = vector<pii>;
using vvp = vector<vp>;
using vt = vector<tii>;
using vvt = vector<vt>;
using mint = modint;
using vm = vector<mint>;
using vvm = vector<vm>;
using mii = map<int64_t, int64_t>;
using pdi = pair<long double, int64_t>;
using tdi = tuple<long double, int64_t, int64_t>;
const int64_t INF = 2e18;
const vp dxy = {{0, -1}, {-1, 0}, {0, 1}, {1, 0}};

template <typename T>
bool chmax(T &a, const T &b) {
  if (a < b) {
    a = b;
    return true;
  }
  return false;
}

template <typename T>
bool chmin(T &a, const T &b) {
  if (a > b) {
    a = b;
    return true;
  }
  return false;
}

int64_t to_int(char c) { return c - 'A'; }

set<char> target = {'A', 'G', 'C', 'T'};

int main() {
  int64_t n;
  cin >> n;
  string s;
  cin >> s;

  int64_t ans = n;
  reverse(all(s));
  for (auto c : s) {
    if (target.count(c)) break;

    ans--;
  }

  cout << ans << endl;
}