#pragma GCC target("avx2")
#pragma GCC optimize("unroll-loops")

#include <algorithm>
#include <bitset>
#include <climits>
#include <cmath>
#include <cstring>
#include <deque>
#include <forward_list>
#include <functional>
#include <iomanip>
#include <iostream>
#include <list>
#include <map>
#include <numeric>
#include <queue>
#include <random>
#include <set>
#include <stack>
#include <string>
#include <unordered_map>
#include <unordered_set>
#include <utility>
#include <vector>

#define ALL(x) (x).begin(), (x).end()
#define PC(x) __builtin_popcount(x)
#define PCL(x) __builtin_popcountll(x)

using namespace std;
typedef long long ll;
typedef unsigned long long ull;
typedef pair<int, int> pii;
typedef pair<ll, ll> pll;
struct edge {
    int to, cost, id;
};
const double PI = 3.14159265358979323846;
const double PI2 = PI * 2.0;
const double EPS = 1E-09;
const ll MOD = 1E+09 + 7; // =998244353;
const ll INFL = 1E18;
const int INFI = 1E09;
const int MAX_N = 2E+05;

ll dx[4] = { 0, -1, 0, 1 }, dy[4] = { 1, 0, -1, 0 };

int N;

string S;

int main()
{
    ios::sync_with_stdio(false);
    cin.tie(nullptr);

    cin >> N >> S;

    int fir = N - 1, last = 0, pos = -1;
    for (int i = 0; i < N; ++i) {
        if (S[i] == 'A' || S[i] == 'G' || S[i] == 'C' || S[i] == 'T') {
            fir = min(fir, i);
            last = max(last, i);
            pos++;
        }
    }

    // for (int i = 0; i < N; i++) {
    //     for (int j = 0; j < N; j++) {
    //         cout << "i = " << i << ", j = " << j << ", dp = " << dp[i][j] << "\n";
    //     }
    // }

    if (pos == -1) {
        cout << 0 << "\n";
    } else {
        cout << last - min(fir, pos) + 1 << "\n";
    }

    return 0;
}