#pragma GCC optimize ("O3")
#pragma GCC target ("sse4")

#include <bits/stdc++.h>
#include <ext/pb_ds/tree_policy.hpp>
#include <ext/pb_ds/assoc_container.hpp>
#include <ext/rope>

using namespace std;
using namespace __gnu_pbds;
using namespace __gnu_cxx;
 
typedef long long ll;
typedef long double ld;
typedef complex<ld> cd;

typedef pair<int, int> pi;
typedef pair<ll,ll> pl;
typedef pair<ld,ld> pd;

typedef vector<int> vi;
typedef vector<ld> vd;
typedef vector<ll> vl;
typedef vector<pi> vpi;
typedef vector<pl> vpl;
typedef vector<cd> vcd;

template <class T> using Tree = tree<T, null_type, less<T>, rb_tree_tag,tree_order_statistics_node_update>;

#define FOR(i, a, b) for (int i = (a); i < (b); i++)
#define F0R(i, a) for (int i = 0; i < (a); i++)
#define FORd(i,a,b) for (int i = (b)-1; i >= (a); i--)
#define F0Rd(i,a) for (int i = (a)-1; i >= 0; i--)
#define trav(a, x) for (auto& a : x)

#define mp make_pair
#define pb push_back
#define f first
#define s second
#define lb lower_bound
#define ub upper_bound

#define sz(x) (int)x.size()
#define beg(x) x.begin()
#define en(x) x.end()
#define all(x) beg(x), en(x)
#define resz resize

const int MOD = 1000000007;
const ll INF = 1e18;
const int MX = 100001;
const ld PI = 4*atan((ld)1);

template<class T> void ckmin(T &a, T b) { a = min(a, b); }
template<class T> void ckmax(T &a, T b) { a = max(a, b); }

namespace io {
    // TYPE ID (StackOverflow)
    
    template<class T> struct like_array : is_array<T>{};
    template<class T, size_t N> struct like_array<array<T,N>> : true_type{};
    template<class T> struct like_array<vector<T>> : true_type{};
    template<class T> bool is_like_array(const T& a) { return like_array<T>::value; }

    // I/O 
    
    void setIn(string s) { freopen(s.c_str(),"r",stdin); }
    void setOut(string s) { freopen(s.c_str(),"w",stdout); }
    void setIO(string s = "") {
        ios_base::sync_with_stdio(0); cin.tie(0);
        if (sz(s)) { setIn(s+".in"), setOut(s+".out"); }
    }
    
    // INPUT 
    
    template<class T> void re(T& x) { cin >> x; }
    template<class Arg, class... Args> void re(Arg& first, Args&... rest);
    void re(double& x) { string t; re(t); x = stod(t); }
    void re(ld& x) { string t; re(t); x = stold(t); }
    
    template<class T> void re(complex<T>& x);
    template<class T1, class T2> void re(pair<T1,T2>& p);
    template<class T> void re(vector<T>& a);
    template<class T, size_t SZ> void re(array<T,SZ>& a);
    
    template<class Arg, class... Args> void re(Arg& first, Args&... rest) { re(first); re(rest...); }
    template<class T> void re(complex<T>& x) { T a,b; re(a,b); x = cd(a,b); }
    template<class T1, class T2> void re(pair<T1,T2>& p) { re(p.f,p.s); }
    template<class T> void re(vector<T>& a) { F0R(i,sz(a)) re(a[i]); }
    template<class T, size_t SZ> void re(array<T,SZ>& a) { F0R(i,SZ) re(a[i]); }
    
    // OUTPUT 
    
    template<class T1, class T2> ostream& operator<<(ostream& os, const pair<T1,T2>& a) {
        os << '{' << a.f << ", " << a.s << '}'; return os;
    }
    template<class T> ostream& printArray(ostream& os, const T& a, int SZ) {
        os << '{';
        F0R(i,SZ) {
            if (i) {
                os << ", ";
                if (is_like_array(a[i])) cout << "\n";
            }
            os << a[i];
        }
        os << '}';
        return os;
    }
    template<class T, size_t SZ> ostream& operator<<(ostream& os, const array<T,SZ>& a) {
        return printArray(os,a,SZ);
    }
    template<class T> ostream& operator<<(ostream& os, const vector<T>& a) {
        return printArray(os,a,sz(a));
    }
    template<class T> ostream& operator<<(ostream& os, const set<T>& a) {
        os << vector<T>(all(a)); return os;
    }
    template<class T1, class T2> ostream& operator<<(ostream& os, const map<T1,T2>& a) {
        os << vector<pair<T1,T2>>(all(a)); return os;
    }
    
    template<class T> void pr(const T& x) { cout << x << '\n'; }
    template<class Arg, class... Args> void pr(const Arg& first, const Args&... rest) { 
        cout << first << ' '; pr(rest...); 
    }
}

using namespace io;

namespace Z {
    vi z(string s) {
        int N = sz(s); s += '#';
        vi ans(N); ans[0] = N;
        while (s[1+ans[1]] == s[ans[1]]) ans[1] ++;
        
        int L = 1, R = ans[1];
        FOR(i,2,N) {
            if (i <= R) ans[i] = min(R-i+1,ans[i-L]);
            while (s[i+ans[i]] == s[ans[i]]) ans[i] ++;
            if (i+ans[i]-1 > R) L = i, R = i+ans[i]-1;
        }
        return ans;
    }
    
    vi get(string a, string b) { // find prefixes of a in b
        string s = a+"@"+b; 
        vi t = z(s);
        return vi(t.begin()+sz(a)+1,t.end());
    }
};

int N,K;
bitset<10001> ok[2001];
string s[2001];
int dp[10001];
string cur, zz;
vi z;

inline int cmp(char a, char b) {
    // assert (a != b);
    if (a < b) return -1;
    return 1;
}

inline string get(pi x) {
    string ret = cur.substr(0,x.f);
    if (x.s) ret += zz;
    return ret;
}

inline int cmp(string a, string b) {
    if (b.substr(0,sz(a)) == a) return 0;
    if (a < b) return -1;
    return 1;
}

inline int cmptest(pi a, pi b) {
    if (a == b) return 0;
    if (a.s == 0) {
        if (b.s == 0 || a.f <= b.f) return 0;
        int com = min(z[sz(zz)+b.f],a.f-b.f);
        if (com == a.f-b.f) return 0;
        return cmp(cur[b.f+com],zz[com]);
    } else {
        if (b.s == 0) {
        	// pr("HI");
	        int com = min(z[sz(zz)+a.f],min(b.f-a.f,sz(zz)));
	        if (com == sz(zz)) return 0;
	        return cmp(zz[com],cur[a.f+com]);
        }
        int com = min(z[sz(zz)+a.f],min(b.f-a.f,sz(zz)));
        if (com == sz(zz)) return 0;
        if (com < b.f-a.f) return cmp(zz[com],cur[a.f+com]);
        com = min(z[b.f-a.f],sz(zz)-(b.f-a.f));
        if (com == sz(zz)-(b.f-a.f)) return 0;
        return cmp(zz[b.f-a.f+com],zz[com]);
    }
}

inline int cmp(pi a, pi b) { 
    return cmptest(a,b);
}

void tri(int ind) {
    zz = s[ind];
    z = Z::z(zz+cur); F0R(i,10000) z.pb(0);
    F0Rd(i,K+1-sz(s[ind])) if (dp[i] == 0) 
        if (dp[i+sz(zz)] == -1 || cmp(mp(i,1),mp(i+sz(s[ind]),0)) == -1) 
        	dp[i+sz(zz)] = 1;
    
    F0R(i,K+1) if (!ok[ind+1][i]) dp[i] = -1;
    vpi vals;
    F0R(i,K+1) if (dp[i] != -1) {
        pi val = {i-sz(zz)*dp[i],dp[i]};
        while (sz(vals) && cmp(vals.back(),val) == 1) vals.pop_back();
        if (sz(vals) && cmp(vals.back(),val) == -1) continue;
        vals.pb(val);
    }
    F0R(i,K+1) dp[i] = -1;
    trav(a,vals) dp[a.f+a.s*sz(zz)] = 0;
    cur = get(vals.back());
}

int main() {
    // you should actually read the stuff at the bottom
    setIO(); re(N,K); F0R(i,N) re(s[i]);
    ok[N][K] = 1;
    F0Rd(i,N) ok[i] = ok[i+1]|(ok[i+1]>>sz(s[i]));
    assert(ok[0][0]);
    FOR(i,1,10001) dp[i] = -1;
    F0R(i,N) tri(i);
    pr(cur);
    // F0R(i,K+1) cout << ok[0][i] << " ";
    // you should actually read the stuff at the bottom
}

/* stuff you should look for
    * int overflow, array bounds
    * special cases (n=1?), set tle
    * do smth instead of nothing and stay organized
*/

Measured memory_bytes: 4268