#include <bits/stdc++.h>

using namespace std;

#define ll long long

const ll MOD = 1e9 + 7, base = 31;

ll hashx[100005], hashn[100005], power[100005];

ll get(int i, int j) {
    return (hashx[j] - hashx[i - 1] * power[j - i + 1] + MOD * MOD) % MOD;
}

ll get_rev(int i, int j) {
    return (hashn[j] - hashn[i - 1] * power[j - i + 1] + MOD * MOD) % MOD;
}

int main()
{
    freopen("Palin.inp", "r", stdin);
    freopen("Palin.out", "w", stdout);

    ios_base::sync_with_stdio(0);
    cin.tie(0);
    cout.tie(0);

    string s;
    cin >> s;

    int k, t;
    cin >> k >> t;

    int n = s.size();
    string r(s.rbegin(), s.rend());
    s = ' ' + s;
    r = ' ' + r;
    power[0] = 1;
    for (int i = 1; i <= n; i++) {
        power[i] = (power[i - 1] * base) % MOD;
        hashx[i] = (hashx[i - 1] * base + s[i] - 'a' + 1);
        hashn[i] = (hashn[i - 1] * base + r[i] - 'a' + 1);
    }
    
    int pos_a, pos_b = -1;
    for (int i = 1; i <= n - k + 1 && t > 0; i++) {
        int j = i + k - 1;
        int p = n - j + 1;
        int q = p + j - i;
        if (get(i, j) == get_rev(p, q)) --t, pos_a = i, pos_b = j;
    }
    if (t > 0 || pos_b == -1) cout << "***";
    else cout << s.substr(pos_a, k);
    return 0;
}

#include<bits/stdc++.h>
#define ll long long
#define ios ios_base::sync_with_stdio(0);cin.tie(0);cout.tie(0);
#pragma GCC optimize("O3","unroll-loops")
#pragma GCC optimize("Ofast")
using namespace std;
bool kt(const string &s)
{
    ll l=0,r=s.size()-1;
    while(l<r)
    {
        if(s[l]!=s[r])
            return false;
        l++;
        r--;
    }
    return true;
}

int main() {
    ios
    string st;
    ll k,i;
    cin>>st>>k>>i;
    vector<string>a;
    for(int j=0;j<=st.size()-k;j++)
    {
        string s=st.substr(j,k);
        if(kt(s))
            a.push_back(s);
    }
    sort(a.begin(),a.end());
    if(i>0&&i<=a.size()) cout<<a[i-1];
    else cout<<"***";
    return 0;
}