#include <bits/stdc++.h>
namespace IO {
static const int IN_BUF = 1 << 23, OUT_BUF = 1 << 23;
inline char myGetchar() {
  static char buf[IN_BUF], *ps = buf, *pt = buf;
  if (ps == pt) {
    ps = buf, pt = buf + fread(buf, 1, IN_BUF, stdin);
  }
  return ps == pt ? EOF : *ps++;
}
template <typename T>
inline bool read(T &x) {
  bool op = 0;
  char ch = IO::myGetchar();
  x = 0;
  for (; !isdigit(ch) && ch != EOF; ch = IO::myGetchar()) {
    op ^= (ch == '-');
  }
  if (ch == EOF) {
    return false;
  }
  for (; isdigit(ch); ch = IO::myGetchar()) {
    x = x * 10 + (ch ^ '0');
  }
  if (op) {
    x = -x;
  }
  return true;
}
inline int readStr(char *s) {
  int n = 0;
  char ch = IO::myGetchar();
  for (; isspace(ch) && ch != EOF; ch = IO::myGetchar())
    ;
  for (; !isspace(ch) && ch != EOF; ch = IO::myGetchar()) {
    s[n++] = ch;
  }
  s[n] = '\0';
  return n;
}
inline void myPutchar(char x) {
  static char pbuf[OUT_BUF], *pp = pbuf;
  struct _flusher {
    ~_flusher() { fwrite(pbuf, 1, pp - pbuf, stdout); }
  };
  static _flusher outputFlusher;
  if (pp == pbuf + OUT_BUF) {
    fwrite(pbuf, 1, OUT_BUF, stdout);
    pp = pbuf;
  }
  *pp++ = x;
}
template <typename T>
inline void print_(T x) {
  if (x == 0) {
    IO::myPutchar('0');
    return;
  }
  static int num[40];
  if (x < 0) {
    IO::myPutchar('-');
    x = -x;
  }
  for (*num = 0; x; x /= 10) {
    num[++*num] = x % 10;
  }
  while (*num) {
    IO::myPutchar(num[*num] ^ '0');
    --*num;
  }
}
template <typename T>
inline void print(T x, char ch = '\n') {
  print_(x);
  IO::myPutchar(ch);
}
inline void printStr_(const char *s, int n = -1) {
  if (n == -1) {
    n = strlen(s);
  }
  for (int i = 0; i < n; ++i) {
    IO::myPutchar(s[i]);
  }
}
inline void printStr(const char *s, int n = -1, char ch = '\n') {
  printStr_(s, n);
  IO::myPutchar(ch);
}
}  // namespace IO
using namespace IO;
const int N = 5005;
int n, k;
char a[N];
int len;
char b[N], ans[N];
void check(int l, int r) {
  int k = 0;
  for (int i = r + 1; i <= n && k < len; ++i) {
    b[++k] = a[i];
  }
  for (int i = n; i && k < len; --i) {
    b[++k] = a[i];
  }
  if (std::lexicographical_compare(b + 1, b + 1 + k, ans + 1, ans + 1 + k)) {
    for (int i = 1; i <= k; ++i) {
      ans[i] = b[i];
    }
  }
}
int main() {
  read(n), read(k);
  readStr(a + 1);
  char c = *std::min_element(a + 1, a + 1 + n);
  int now = 0, mx = 0;
  for (int i = 1; i <= n; ++i) {
    ++now;
    if (i == n || a[i] != a[i + 1]) {
      if (a[i] == c) {
        mx = std::max(mx, now);
      }
      now = 0;
    }
  }
  if (k > 30 || (1ll << (k - 1)) * mx >= n) {
    for (int i = 1; i <= n; ++i) {
      IO::myPutchar(c);
    }
    IO::myPutchar('\n');
    return 0;
  }
  len = n - mx * (1 << (k - 1));
  for (int i = 1; i <= len; ++i) {
    ans[i] = 'z' + 1;
  }
  now = 0;
  for (int i = 1; i <= n; ++i) {
    ++now;
    if (a[i] != a[i + 1]) {
      if (a[i] == c && now == mx) {
        check(i - now + 1, i);
      }
      now = 0;
    }
  }
  for (int i = 1; i <= n - len; ++i) {
    IO::myPutchar(c);
  }
  for (int i = 1; i <= len; ++i) {
    IO::myPutchar(ans[i]);
  }
  IO::myPutchar('\n');
}
