#define _GLIBCXX_DEBUG //遅い可能性がある 
#include <bits/stdc++.h>
#include<atcoder/all>
#include<boost/multiprecision/cpp_int.hpp> //多倍長
using namespace boost::multiprecision; // 多倍長.型はcpp_int
using namespace atcoder;
using namespace std;
#define rep(i, n) for (int i = 0; i < (int)(n); i++)
#define ALL(a) (a).begin(),(a).end() //lower_bound(ALL(a),x)
using ll = long long;
using P = pair<int,int>;
using VI = vector<int>;
using VVI = vector<vector<int>>;

int INF = 100100100;

const vector<int> dx = {1, 0, -1, 0};
const vector<int> dy = {0, 1, 0, -1};

int depth[110000];
int mxdepth[110000];

void dfs(VVI& G, int v, int p) {
    for (int u: G[v]) {
        if (u == p) continue;
        depth[u] = depth[v] + 1;
        mxdepth[u] = max(mxdepth[u], depth[u]);
        dfs(G,u,v);
        mxdepth[v] = max(mxdepth[v], mxdepth[u]);
    }
}

int main() {
    int n;
    cin >> n;
    string s;
    cin >> s;
    int ans = 0;
    rep(i,n) {
        if (s[i] == 'A' || s[i] == 'G' || s[i] == 'C' || s[i] == 'T') {
            ans = i+1; // 1-index
        }
    }
    cout << ans << endl;
    return 0;
}