#include <bits/stdc++.h>
/*
#include <boost/multiprecision/cpp_dec_float.hpp>
#include <boost/multiprecision/cpp_int.hpp>
namespace mp = boost::multiprecision;
using bint = mp::cpp_int;
*/
#include <atcoder/all>
#define rep(i,n) for (int i = 0; i < int(n); ++i)
#define repp(i,n,m) for (int i = m; i < int(n); ++i)
#define repb(i,n) for (int i = int(n)-1; i >= 0; --i)
#define endl "\n"
using namespace std;
using namespace atcoder;
using ll = long long;
using ld = long double;
using P = pair<int, int>;
using PL = pair<long long, long long>;
using pdd = pair<long double, long double>;
using pil = pair<int,ll>;
using pli = pair<ll,int>;
using ppi = pair<P,int>;
using pip = pair<int,P>;
const int INF = 1001001007;
const long long mod1 = 1000000007LL;
const long long mod2 = 998244353LL;
const ll inf = 2e18;
const ld pi = 3.14159265358979323;
const ld eps = 1e-7;
const char _ = ' ';
template<typename T>void o(T a);
template<class T>istream &operator>>(istream &is,vector<T> &v){for(auto &e:v)is>>e;return is;}
template<class T>ostream &operator<<(ostream &os,const vector<T> &v){if(v.size()!=0){rep(i,v.size())os<<v[i]<<(i+1==v.size()?"":" ");}return os;}
template<class T>istream &operator>>(istream &is,vector<vector<T>> &v){for(auto &e:v)is>>e;return is;}
template<class T>ostream &operator<<(ostream &os,const vector<vector<T>> &v){if(v.size()!=0){for(auto &e:v)o(e);}return os;}
template<typename T>bool range(T a,T b,T x){return (a<=x&&x<b);}
template<typename T>bool rrange(T a,T b,T c,T d,T x,T y){return (range(a,c,x)&&range(b,d,y));}
template<typename T>void rev(vector<T> &v){reverse(v.begin(),v.end());}
void revs(string &s) {reverse(s.begin(),s.end());}
template<typename T>void sor(vector<T> &v, int f=0){sort(v.begin(),v.end());if(f!=0) rev(v);}
template<typename T>bool chmin(T &a,const T &b){if(a>b){a=b;return true;}return false;}
template<typename T>bool chmax(T &a,const T &b){if(a<b){a=b;return true;}return false;}
template<typename T>void eru(vector<T> &v){sor(v);v.erase(unique(v.begin(),v.end()),v.end());}
template<typename T>T cel(T a,T b){if(a%b==0)return a/b;return a/b +1;}
void o(){cout<<"!?"<<endl;}
template<typename T>void o(T a){cout<<a<<endl;}

template<typename T>void mout(T a){cout<<a.val()<<endl;}
void print(){ putchar(' '); }
void print(bool a){ printf("%d", a); }
void print(int a){ printf("%d", a); }
void print(long a){ printf("%ld", a); }
void print(long long a){ printf("%lld", a); }
void print(char a){ printf("%c", a); }
void print(char a[]){ printf("%s", a); }
void print(const char a[]){ printf("%s", a); }
void print(long double a){ printf("%.15Lf", a); }
void print(const string& a){ for(auto&& i : a) print(i); }
template<class T> void print(const T& a){ cout << a; }
int out(){ putchar('\n'); return 0; }
template<class T> int out(const T& t){ print(t); putchar('\n'); return 0; }
template<class Head, class... Tail> int out(const Head& head, const Tail&... tail){ print(head); putchar(' '); out(tail...); return 0; }
void yes(){cout << "Yes" << endl;}
void no (){cout << "No" << endl;}
void yn (bool t){if(t)yes();else no();}
template<typename T>void dame(bool t, T s){if(!t){cout << s << endl;exit(0);}}
void fast_io(){cin.tie(0); ios::sync_with_stdio(0); cout<<fixed<<setprecision(20);}
template<typename T,typename U>void o2(pair<T,U> a){out(a.first,a.second);}
vector<int> dx = {0,1,0,-1,1,1,-1,-1};
vector<int> dy = {1,0,-1,0,1,-1,-1,1};
const string ALP = "ABCDEFGHIJKLMNOPQRSTUVWXYZ";
const string alp = "abcdefghijklmnopqrstuvwxyz";
const string NUM = "0123456789";

ll gcd(ll a,ll b){return b?gcd(b,a%b):a;}
ll lcm(ll a,ll b){return a/gcd(a,b)*b;}
ll mpow(ll x,ll n,ll m){if(n==0)return 1LL;x%=m;ll a=mpow(x,n/2,m);a=a*a%m;return (n&1)?a*x%m:a;}

void solve(){
    int n; cin >> n;
    string s; cin >> s;
    vector<char> del = {'A','C','G','T'};
    deque<int> le, ri;
    vector<int> a(n,0);
    int cnt = 0;
    rep(i,n) for (char c : del) if (c == s[i]) a[i]++, cnt++;
    int ans = 0;
    rep(i,cnt-1) le.push_back(a[i]);
    repp(i,n,cnt-1) ri.push_back(a[i]);
    while (cnt > 0){
        int p = ri.front(); ri.pop_front();
        ans++;
        if (p == 0) continue;
        cnt--;
        if (cnt > 0) ri.push_front(le.back()), le.pop_back();
    }
    o(ans);
}


int main(){
    fast_io();
    int t = 1; // cin >> t;
    while (t--) solve();
}