#include <bits/stdc++.h>
using namespace std;
void fastIO() {
  std::ios_base::sync_with_stdio(0);
  cin.tie(0);
  cout.tie(0);
}
template <typename T, typename U>
inline void mnze(T &x, U y) {
  if (y < x) x = y;
}
template <typename T, typename U>
inline void mxze(T &x, U y) {
  if (x < y) x = y;
}
void _scan(int &x) { scanf("%d", &x); }
void _scan(long long &x) { scanf("%lld", &x); }
void _scan(double &x) { scanf("%lf", &x); }
void _scan(char &x) { scanf(" %c", &x); }
void _scan(char *x) { scanf("%s", x); }
template <typename T>
void _scan(vector<T> &v) {
  for (int i = 0; i < (int)v.size(); i++) _scan(v[i]);
}
void scan() {}
template <typename T, typename... U>
void scan(T &head, U &...tail) {
  _scan(head);
  scan(tail...);
}
template <typename T>
void _dbg(const char *sdbg, T h) {
  cerr << sdbg << "=" << h << "\n";
}
template <typename T, typename... U>
void _dbg(const char *sdbg, T h, U... t) {
  while (*sdbg != ',') cerr << *sdbg++;
  cerr << "=" << h << ",";
  _dbg(sdbg + 1, t...);
}
long long MOD = 1e9 + 7;
inline long long Msum(long long x) { return x; }
template <typename... Rest>
inline long long Msum(long long x, Rest... rest) {
  return (x + Msum(rest...)) % MOD;
}
inline long long Mprod(long long x) { return x; }
template <typename... Rest>
inline long long Mprod(long long x, Rest... rest) {
  return x * Mprod(rest...) % MOD;
}
inline long long Mnorm(long long x) { return (x % MOD + MOD) % MOD; }
inline long long Msq(long long x) { return (x * x) % MOD; }
inline long long Mpot(long long x, long long e) {
  if (!e) return 1;
  return Msq(Mpot(x, e >> 1)) * (e & 1 ? x : 1) % MOD;
}
const int MAXN = 3e5 + 5;
int n, k;
char s[MAXN];
char ans[MAXN];
int main() {
  scan(n, k, s);
  long long pot = 1;
  for (int i = 0; i < k - 1; i++) {
    pot *= 2;
    if (pot >= n) break;
  }
  char mn = s[0];
  for (int i = 0; i < n; i++) mnze(mn, s[i]);
  int cnt = 0, len = 0;
  for (int i = 0; i < n; i++) {
    if (s[i] == mn)
      cnt++;
    else
      cnt = 0;
    mxze(len, cnt);
  }
  if (pot * len >= n) {
    for (int i = 0; i < n; i++) ans[i] = mn;
    printf("%s\n", ans);
    return 0;
  }
  for (int i = 0; i < pot * len; i++) ans[i] = mn;
  for (int i = pot * len; i < n; i++) ans[i] = 'z';
  cnt = 0;
  for (int i = 0; i < n; i++) {
    if (s[i] == mn)
      cnt++;
    else
      cnt = 0;
    if (cnt == len) {
      int dir = 1;
      int j = i + 1;
      bool lss = false;
      for (int k = pot * len; k < n; k++) {
        if (j == n) {
          j = n - 1;
          dir = -1;
        }
        if (lss) {
          ans[k] = s[j];
        } else {
          if (s[j] < ans[k]) {
            lss = true;
            ans[k] = s[j];
          } else if (s[j] > ans[k]) {
            break;
          }
        }
        j = j + dir;
      }
    }
  }
  printf("%s\n", ans);
}
