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

#define fs first
#define sc second
#define pb emplace_back
#define mp make_pair
#define all(v) v.begin(), v.end()
#define rall(v) v.rbegin(), v.rend()

constexpr int mod = 1e9 + 7;
constexpr int32_t inf = 1001001001;
constexpr int64_t infll = 1001001001001001001ll;
constexpr int dx[] = {0, -1, 1, 0, -1, 1, -1, 1}, dy[] = {-1, 0, 0, 1, -1, -1, 1, 1};
const string YES = "YES", NO = "NO", Yes = "Yes", No = "No", yes = "yes", no = "no";

// clang-format off
// ostream &operator<<(ostream &os, __int128_t value) { if (ostream::sentry(os)) { __uint128_t tmp = value < 0 ? -value : value; char buffer[64]; char *d = end(buffer); do { --d; *d = "0123456789"[tmp % 10]; tmp /= 10; } while (tmp != 0); if (value < 0) { --d; *d = '-'; } int len = end(buffer) - d; if (os.rdbuf()->sputn(d, len) != len) { os.setstate(ios_base::badbit); }} return os; }
// istream &operator>>(istream &is, __int128_t &value) { string in; is >> in; value = 0; for (const char &c : in) { if ('0' <= c && c <= '9') value = 10 * value + (c - '0'); } if (in[0] == '-') value *= -1; return is; }
// ostream &operator<<(ostream &os, __uint128_t value) { if (ostream::sentry(os)) { char buffer[64]; char *d = end(buffer); do { --d; *d = "0123456789"[value % 10]; value /= 10; } while (value != 0); int len = end(buffer) - d; if (os.rdbuf()->sputn(d, len) != len) { os.setstate(ios_base::badbit); }} return os; }
// istream &operator>>(istream &is, __uint128_t &value) { string in; is >> in; value = 0; for (const char &c : in) { if ('0' <= c && c <= '9') value = 10 * value + (c - '0'); } return is; }
template <typename T> ostream &operator<<(ostream &os, vector<T> &v) { os << v[0]; for (int i = 1; i < v.size(); ++i) os << " " << v[i]; return os; }
template <typename T> istream &operator>>(istream &is, vector<T> &v) { for (T &in : v) is >> in; return is; }
template <typename T1, typename T2> ostream &operator<<(ostream &os, pair<T1, T2> p) { os << p.fs << " " << p.sc; return os; }
template <typename T1, typename T2> istream &operator>>(istream &is, pair<T1, T2> &p) { is >> p.fs >> p.sc; return is; }

struct IoSetup { IoSetup(){ cin.tie(0); ios::sync_with_stdio(0); cout << fixed << setprecision(10); cerr << fixed << setprecision(10); } } iosetup;

inline int64_t in() { int64_t x = 0; cin >> x; return x; }

template <typename T> void sort(vector<T> &v) { sort(all(v)); }
template <typename T> vector<T> make_v(size_t a, T b) { return vector<T>(a, b); }
template <typename... Ts> auto make_v(size_t a, Ts... ts) { return vector<decltype(make_v(ts...))>(a, make_v(ts...)); }
template <typename T1, typename T2> inline bool chmax(T1 &a, T2 b) { return a < b && (a = b, true); }
template <typename T1, typename T2> inline bool chmin(T1 &a, T2 b) { return a > b && (a = b, true); }
// clang-format on

vector<int> Zalgorithm(string s) {
  vector<int> res(s.size());
  res[0] = s.size();
  int i = 1, j = 0;
  while (i < s.size()) {
    while (i + j < s.size() && s[j] == s[i + j]) ++j;
    res[i] = j;
    if (j == 0) {
      ++i;
      continue;
    }
    int k = 1;
    while (i + k < s.size() && k + res[k] < j) res[i + k] = res[k], ++k;
    i += k;
    j -= k;
  }

  return res;
}

signed main(int argc, char *argv[]) {
  int n = in(), k = in();
  vector<string> s(n + 1);
  for (int i = 1; i <= n; ++i) {
    cin >> s[i];
  }

  vector<bitset<10101>> reach(n + 1);
  reach.back().set(k);
  for (int i = n; i >= 1; --i) {
    reach[i - 1] = reach[i] | (reach[i] >> s[i].size());
  }

  auto dp = make_v(n + 1, k + 10000, false);
  dp[0][0] = true;

  string ans;
  for (int i = 1; i <= n; ++i) {
    dp[i] = dp[i - 1];

    vector<int> d = Zalgorithm(s[i] + ans);
    for (int j = 0; j <= ans.size() && j + s[i].size() <= k; ++j) {
      int t = (j == ans.size() ? 0 : d[s[i].size() + j]);

      if (dp[i][j] && reach[i][j + s[i].size()] && (j == ans.size() || (t < s[i].size()) && (ans[j + t] > s[i][t]))) {
        ans = ans.substr(0, j) + s[i];
        for (int l = j + t + 1; l <= k; ++l) dp[i][l] = false;
        dp[i][j + s[i].size()] = true;
        break;
      }
    }
  }

  cout << ans << endl;
}


Measured memory_bytes: 8960