#include <bits/stdc++.h>

using namespace std;
typedef long long ll;
typedef long double ld;
#define rep(i, a, n) for (int i = (a); i < (n); i++)
#define per(i, a, n) for (int i = (n) - 1; i >= (a); i--)

typedef pair<ll,int> pli;
struct SuffixArray {
    vector<int> a;
    string s;
    SuffixArray(const string& _s) : s(_s + '\0') {
        int N = s.size();
        vector<pli> b(N);
        a.resize(N);
        rep(i,0,N) {
            b[i].first = s[i];
            b[i].second = i;
        }
        int q = 8;
        while ((1<<q) < N) q++;
        for (int moc = 0;; moc++) {
            // count_sort()
            sort(b.begin(), b.end());
            a[b[0].second] = 0;
            rep(i,1,N)
                a[b[i].second] = a[b[i-1].second] + (b[i-1].first != b[i].first);
            if ((1 << moc) >= N) break;
            rep(i,0,N) {
                b[i].first = (ll)a[i] << q;
                if (i + (1 << moc) < N)
                    b[i].first += a[i + (1 << moc)];
                b[i].second = i;
            }
        }
        rep(i,0,a.size()) a[i] = b[i].second;
    }
};
int n, k;

bool isOk(const vector<int>& sai, int lim, int want) {
    int n = sai.size();
    vector<bool> allowed(n);
    rep(i,0,n) {
        allowed[i] = sai[i] <= lim;
    }
    //rep(i,0,n) cout << allowed[i];
    //cout << endl;

    int fuel = 0;
    int dist = 0;
    rep(i,0,n) {
        //cout << i << " " << dist << " " << fuel << endl;
        dist++;
        fuel--;
        if (allowed[i]) {
            fuel = k-1;
        }
        if (fuel >= 0) {
            if (dist >= want) {
                //cout << "yes" << endl;
                return true;
            }

        } else {
            dist = 0;
        }

    }
    //cout << "no" << endl;
    return false;
}

int main(void) {
	ios_base::sync_with_stdio(false);
	cin >> n >> k;
	string s0;
	cin >> s0;
	string s = s0 + s0;

	SuffixArray sarr(s);
	//cout << s << endl;

	vector<int> sai(sarr.a.size());
	rep(i,0,sarr.a.size()) {
        sai[sarr.a[i]] = i;
	}
	//for(auto x : sarr.a) cout << x << endl;

	int fr = 0, to = 2*n;
	int best = -1;
	while (fr <= to) {
        //cout << fr << " " << to << endl;
        int mid = (fr + to) / 2;
        //cout  << s << endl;
        if (isOk(sai, mid, n)) {
            best = mid;
            to = mid - 1;
        } else {
            fr = mid + 1;
        }
	}
	assert(best != -1);
	//cout << best << " " << sarr.a[best] << endl;
	cout << s.substr((sarr.a[best]) % n, k) << endl;
}