#include <bits/stdc++.h>
using namespace std;
int main()
{
    string s, res = "";
    int n, ans = -1;
    cin >> s >> n;
    for (int i = 0; i < s.size(); i++)
    {
        int cnt = 0;
        string tmp = "";
        for (int j = i; j < s.size() && j < i + n; j++)
        {
            tmp += s[j];
            if (s[j] == 'G' || s[j] == 'C') cnt++;
        }
        if (cnt > ans) res = tmp, ans = cnt;
    }
    cout << res << endl;
}