/**
 *    author:   Binoy Barman
 *    created:  2025-08-29 11:00:07
**/

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

int32_t main() {
    ios::sync_with_stdio(false); 
    cin.tie(nullptr);

    int n, k;
    cin >> n >> k;
    string s;
    cin >> s;

    vector<int> f(26, 0);
    int i = 0, j = 0;
    while(i < n) {
        int r = i + k - 1;
        if(r >= n) break; // not enough length for a k-window
        if(r == n - 1) {
            sort(s.begin() + i, s.begin() + r + 1);
            break;
        }

        // build frequency for substring [i .. r]
        while(j < r) {
            f[s[j] - 'a']++;
            j++;
        }
        f[s[j] - 'a']++;
        
        // check if there exists a smaller character than s[i]
        int cur = s[i] - 'a';
        bool exists = false;
        for (int id = 0; id < cur; id++) {
            exists |= (f[id] > 0);
        }

        if(exists) {
            // sorting this window is optimal
            sort(s.begin() + i, s.begin() + r + 1);
            while(i <= r) {
                f[s[i] - 'a']--;
                i++;
            }
        }
        else {
            f[cur]--;
            i++;
        }
    }
    cout << s << '\n';
    return 0;
}