#include<algorithm>
#include<iostream>
#include<cstring>
#include<vector>
using namespace std;
typedef long long lint;
typedef vector<int>vi;
typedef pair<int,int>pii;
#define rep(i,n)for(int i=0;i<(int)(n);++i)
#define REP(i,n) for(int i=0;i<(int)n;++i)
#define FOR(i,c) for(__typeof((c).begin())i=(c).begin();i!=(c).end();++i)

const int DEBUG=0;

// http://www.prefield.com/algorithm/string/suffix_array.html

// Larsson-Sadakane's Suffix array Construction: O(n (log n)^2)
struct SAComp {
  const int h, *g;
  SAComp(int h, int* g) : h(h), g(g) {}
  bool operator() (int a, int b) {
    return a == b ? false : g[a] != g[b] ? g[a] < g[b] : g[a+h] < g[b+h];
  }
};
int *buildSA(const char* t, int n) {
  int g[n+1], b[n+1], *v = new int[n+1];
  REP(i,n+1) v[i] = i, g[i] = t[i];
  b[0] = 0; b[n] = 0;

  sort(v, v+n+1, SAComp(0, g));
  for(int h = 1; b[n] != n ; h *= 2) {
    SAComp comp(h, g);
    sort(v, v+n+1, comp);
    REP(i, n) b[i+1] = b[i] + comp(v[i], v[i+1]);
    REP(i, n+1) g[v[i]] = b[i];
  }
  return v;
}
// Naive matching O(m log n)
int find(char *t, int n, char *p, int m, int *sa) {
  int a = 0, b = n;
  while (a < b) {
    int c = (a + b) / 2;
    if (strncmp(t+sa[c], p, m) < 0) a = c+1; else b = c;
  }
  return strncmp(t+sa[a], p, m) == 0 ? sa[a] : -1;
}

// Kasai-Lee-Arimura-Arikawa-Park's simple LCP computation: O(n)
int *buildLCP(char *t, int n, int *a) {
  int h = 0, b[n+1], *lcp = new int[n+1];
  REP(i, n+1) b[a[i]] = i;
  REP(i, n+1) {
    if (b[i]){
      for (int j = a[b[i]-1]; j+h<n && i+h<n && t[j+h] == t[i+h]; ++h);
      lcp[b[i]] = h;
    } else lcp[b[i]] = -1;
    if (h > 0) --h;
  }
  return lcp;
}
// call RMQ = buildRMQ(lcp, n+1)
int *buildRMQ(int *a, int n) {
  int logn = 1;
  for (int k = 1; k < n; k *= 2) ++logn;
  int *r = new int[n * logn];
  int *b = r; copy(a, a+n, b);
  for (int k = 1; k < n; k *= 2) {
    copy(b, b+n, b+n); b += n;
    REP(i, n-k) b[i] = min(b[i], b[i+k]);
  }
  return r;
}
// inner LCP computation with RMQ: O(1)
int minimum(int x, int y, int *rmq, int n) {
  int z = y - x, k = 0, e = 1, s; // y - x >= e = 2^k なる最大 k
  s = ( (z & 0xffff0000) != 0 ) << 4; z >>= s; e <<= s; k |= s;
  s = ( (z & 0x0000ff00) != 0 ) << 3; z >>= s; e <<= s; k |= s;
  s = ( (z & 0x000000f0) != 0 ) << 2; z >>= s; e <<= s; k |= s;
  s = ( (z & 0x0000000c) != 0 ) << 1; z >>= s; e <<= s; k |= s;
  s = ( (z & 0x00000002) != 0 ) << 0; z >>= s; e <<= s; k |= s;
  return min( rmq[x+n*k], rmq[y+n*k-e+1] );
}
// outer LCP computation: O(m - o)
int computeLCP(char *t, int n, char *p, int m, int o, int k) {
  int i = o;
  for (; i < m && k+i < n && p[i] == t[k+i]; ++i);
  return i;
}
// Mamber-Myers's O(m + log n) string matching with LCP/RMQ
#define COMP(h, k) (h == m || (k+h<n && p[h]<t[k+h]))
int find(char *t, int n, char *p, int m, int *sa, int *rmq) {
  int l = 0, lh = 0, r = n, rh = computeLCP(t,n+1,p,m,0,sa[n]);
  if (!COMP(rh, sa[r])) return -1;
  for (int k = (l+r)/2; l+1 < r; k = (l+r)/2) {
    int A = minimum(l+1, k, rmq, n+1), B = minimum(k+1, r, rmq, n+1);
    if (A >= B) {
      if (lh < A) l = k;
      else if (lh > A) r = k, rh = A;
      else {
        int i = computeLCP(t, n+1, p, m, A, sa[k]);
        if (COMP(i, sa[k])) r = k, rh = i; else l = k, lh = i;
      }
    } else {
      if (rh < B) r = k;
      else if (rh > B) l = k, lh = B;
      else {
        int i = computeLCP(t, n+1, p, m, B, sa[k]);
        if (COMP(i, sa[k])) r = k, rh = i; else l = k, lh = i;
      }
    }
  }
  return rh == m ? sa[r] : -1;
}

int main(){
  int n;
  lint h;
  string s;
  cin>>n>>h>>s;
  vi multa(n+1);
  int cnt=0;
  rep(i,n+1){
    cnt++;
    if(i<n&&s[i]=='0'){
      multa[i]=0;
    }else{
      multa[i]=cnt;
      cnt=0;
    }
  }
  int lo=-1,hi=n;
  while(hi-lo>1){
    int x=(hi+lo)/2;
    lint cnt=0;
    rep(i,n+1){
      if(i<n)
	cnt+=(lint)min(x,n-i)*multa[i];
      cnt+=(lint)multa[i]*(lint)(multa[i]-1)/2;
    }
    if(cnt>=h)
      hi=x;
    else
      lo=x;
  }
  int *sa=buildSA(s.c_str(),n);
  if(DEBUG){
    rep(i,n+1){
      cerr<<"suf:"<<sa[i]<<" "<<s.substr(sa[i])<<endl;
    }
  }
  int len=hi;
  if(DEBUG)cerr<<"len="<<len<<endl;
  vi tak;
  rep(i,n+1){
    if(sa[i]<=n-len)tak.push_back(sa[i]);
  }
  lint base=0;
  rep(i,n+1){
    if(i<n)
      base+=(lint)min(len-1,n-i)*multa[i];
    base+=(lint)multa[i]*(lint)(multa[i]-1)/2;
  }
  if(len==0){
    cout<<0<<endl;
    return 0;
  }
  if(DEBUG)
    cerr<<"base="<<base<<endl;
  if(DEBUG){
    cerr<<"tak:";
    rep(i,tak.size()){
      cerr<<" "<<tak[i];
      //cerr<<" "<<s.substr(tak[i])<<endl;
    }
    cerr<<endl;
  }
  int tmp=0,idx=-1;
  rep(i,tak.size()){
    if(s[tak[i]]!='0'){
      tmp+=multa[tak[i]];
      if(tmp+base>=h){
	idx=tak[i];
	break;
      }
    }
  }
  cout<<s.substr(idx,len)<<endl;
}



Measured memory_bytes: 5052