#include <bits/stdc++.h>
using namespace std;
#define repu(i, a, b) for (int i = (a); i < (b); ++i)
#define repd(i, a, b) for (int i = (a); i > (b); --i)
#define mem(a, x) memset(a, x, sizeof(a))
#define all(a) a.begin(), a.end()
#define uni(a) a.erase(unique(all(a)), a.end())
#define count_bits(x) __builtin_popcount(x)
#define count_bitsll(x) __builtin_popcountll(x)
#define least_bits(x) __builtin_ffs(x)
#define least_bitsll(x) __builtin_ffsll(x)
#define most_bits(x) 32 - __builtin_clz(x)
#define most_bitsll(x) 64 - __builtin_clz(x)
vector<string> split(const string &s, char c) {
vector<string> v;
stringstream ss(s);
string x;
while (getline(ss, x, c)) v.push_back(x);
return v;
}
#define error(args...) { vector<string> _v = split(#args, ','); err(_v.begin(), args); }
void err(vector<string>::iterator it) {}
template<typename T, typename... Args>
void err(vector<string>::iterator it, T a, Args... args) {
cerr << it -> substr((*it)[0] == ' ', it -> length()) << " = " << a << '\n';
err(++it, args...);
}
typedef long long ll;
const int MOD = 1000000007;
template<class T> inline T tmin(T a, T b) {return (a < b) ? a : b;}
template<class T> inline T tmax(T a, T b) {return (a > b) ? a : b;}
template<class T> inline void amax(T &a, T b) {if (b > a) a = b;}
template<class T> inline void amin(T &a, T b) {if (b < a) a = b;}
template<class T> inline T tabs(T a) {return (a > 0) ? a : -a;}
template<class T> T gcd(T a, T b) {while (b != 0) {T c = a; a = b; b = c % b;} return a;}
int main(int argc, char *argv[]) {
ios_base::sync_with_stdio(false);
string s;
int n;
cin >> s >> n;
vector<string> ret;
repu(i, 0, 5) repu(j, 0, 5) {
string tmp = "";
tmp += s[i]; tmp += s[j];
ret.push_back(tmp);
}
sort(all(ret));
cout << ret[n - 1] << endl;
return 0;
}