#include <iostream>
#include <bits/stdc++.h>
using namespace std;
#define mp make_pair
#define REP(A,B) for(int (A)=0;(A)<(B);(A)++)
#define ll long long

#define lld long double
typedef pair<ll, int> pli;
#define pb push_back
typedef vector<int> vi;

struct SuffixArray {
	vi a;
	string s;
	SuffixArray(const string& _s) : s(_s + '\0') {
		int N = s.size();
		vector<pli> b(N);
		a.resize(N);
		REP(i, N) {
			b[i].first = s[i];
			b[i].second = i;
		}
		int q = 8;
		while((1<<q) < N) q++;
		for(int moc = 0; ; moc++) {
			sort(b.begin(), b.end());
			a[b[0].second] = 0;
			for(int i = 1; i < N; i++) {
				a[b[i].second] = a[b[i-1].second]+(b[i-1].first!=b[i].first);
			}
			if((1<<moc) >= N)
				break;
			REP(i, 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, a.size()) a[i] = b[i].second;
	}
};
char initS[555555];
int hodn[555555];
int main() {
	int n,k;
	scanf("%d%d", &n, &k);
	scanf("%s", initS);
	string S = string(initS);
	S += S;
	vector<int> arr = SuffixArray(S).a;
	vector<int> oarr;
	int poradi = 0;
	REP(i, 2*n+1) {
		if(arr[i] < n) {
			hodn[arr[i]] = poradi++;
			oarr.pb(arr[i]);
		}
	}
	set< pair<int,int> > sset;
	REP(i, k-1) {
		sset.insert(mp(hodn[(-1-i+n)%n], (-1-i+n)%n));
	}
	int ans = -1;
	int ansi = -1;
	REP(i, n) {
		//printf("ins %d\n", i);
		sset.insert(mp(hodn[i], i));
		auto it = sset.begin();
		//printf("i=%d, it->first = %d\n", i, it->first);
		if(it->first > ans) {
			ans = max(ans, it->first);
			ansi = oarr[it->first];

		//	printf("new maximum at i=%d: (%d, %d)\n", i, it->first, it->second);
		}
		sset.erase(sset.find(mp(hodn[(i-k+1+n)%n], (i-k+1+n)%n)));
		//printf("del %d\n", (i-k+1+n)%n);
	}
	REP(i, k) printf("%c", S[(ansi+i)%n]);
	printf("\n");
	return 0;
}