#include <bits/stdc++.h>
using namespace std;
typedef long long ll;

signed main() {
    ios_base::sync_with_stdio(0); cin.tie(0);
    ll n, k; cin >> n >> k;
    string s; cin >> s;

    if (n == k) {
        sort(s.begin(), s.end());
        cout << s << "\n";
        return 0;
    }

    ll indx[n];
    for (ll i = 0; i < n; i++) indx[i] = i;

    ll j = n-1;
    set<pair<char, ll>> st;
    for (ll i = n-1; i >= 0; i--) {
        if (st.size() >= k) {
            st.erase({s[j], j});
            j--;
        }
        st.insert({s[i], i});

        auto it = *st.begin();
        if (it.first < s[i]) indx[i] = it.second;
    }

    ll ache = 0;
    for (ll i = 0; i < n; ) {
        if (indx[i] == i) {
            cout << s[i];
            ache++;
            i++;
            continue;
        }

        if (i+k >= n) {
            if (ache + (n-i) >= k) {
                string temp = s.substr(i);
                sort(temp.begin(), temp.end());
                cout << temp;
            }  
            else cout << s.substr(i);
            break;
        }

        string temp = s.substr(i, k);
        sort(temp.begin(), temp.end());
        cout << temp;
        i += k;
        ache = 0;
    }
}