#include <algorithm>
#include <cmath>
#include <iostream>
#include <iterator>
#include <map>
#include <string>
#include <vector>
using namespace std;
int main() {
string seq;
int sub_str_len;
cin >> seq >> sub_str_len;
int seq_len = seq.size();
vector<double> ratios;
vector<string> sub_strs;
for (int i = 0; i <= (seq_len - sub_str_len); i ++) {
// 1. 依次取出子串
string temp = seq.substr(i, sub_str_len);
// 2. 在子串中算出 GC 比例
int counter = 0;
for (char ch : temp) {
if (ch == 'C' || ch == 'G') {
counter ++;
}
}
double ratio = double(counter) / double(sub_str_len);
// 3. 将比例、子串对 存入vector中
ratios.push_back(ratio);
sub_strs.push_back(temp);
}
// 4. 找的比例的最大值
auto max_pos = max_element(ratios.begin(), ratios.end());
double max_ratio = *max_pos;
// 5.遍历ratios 找到最大值第一次出现的下标
double eps = pow(10, -4);
int index = 0;
for (int i = 0; i < ratios.size(); i ++) {
if (abs(ratios[i] - max_ratio) <= eps) {
index = i;
break;
}
}
// 6. 输出sub_strs在index的内容
cout << sub_strs[index] << endl;
return 0;
}
// 64 位输出请用 printf("%lld")