#include<cstdio>

#include<cstring>

#include<algorithm>

#define N 100010

using namespace std;

typedef long long ll;

char s[N];

int K,n,i,j,rk[N],sa[N],height[N],tmp[N],cnt[N],Log[N],f[16][N],g[N];

ll L=1,R,mid;

int len,nowl,nowr,ansl,ansr;

void suffixarray(int n,int m){

  int i,j,k;n++;

  for(i=0;i<n;i++)cnt[rk[i]=s[i]]++;

  for(i=1;i<m;i++)cnt[i]+=cnt[i-1];

  for(i=0;i<n;i++)sa[--cnt[rk[i]]]=i;

  for(k=1;k<=n;k<<=1){

    for(i=0;i<n;i++){

      j=sa[i]-k;

      if(j<0)j+=n;

      tmp[cnt[rk[j]]++]=j;

    }

    sa[tmp[cnt[0]=0]]=j=0;

    for(i=1;i<n;i++){

      if(rk[tmp[i]]!=rk[tmp[i-1]]||rk[tmp[i]+k]!=rk[tmp[i-1]+k])cnt[++j]=i;

      sa[tmp[i]]=j;

    }

    memcpy(rk,sa,n*sizeof(int));

    memcpy(sa,tmp,n*sizeof(int));

    if(j>=n-1)break;

  }

  for(j=rk[height[i=k=0]=0];i<n-1;i++,k++)

    while(~k&&s[i]!=s[sa[j-1]+k])height[j]=k--,j=rk[sa[j]+1];

}

inline int lcp(int x,int y){

  if(x==y)return n-x;

  x=rk[x],y=rk[y];

  if(x>y)swap(x,y);

  int k=Log[y-x];

  return min(f[k][x+1],f[k][y-(1<<k)+1]);

}

void kth(ll k){

  ll s=0;

  for(int i=1;i<=n;s+=n-sa[i]-height[i],i++)if(s+n-sa[i]-height[i]>=k){

    nowl=sa[i],nowr=nowl+height[i]+k-s-1;

    len=nowr-nowl+1;

    return;

  }

}

inline bool ask(int l,int r){

  int t=min(lcp(l,nowl),min(r-l+1,len));

  if(t==r-l+1&&t<=len)return 1;

  if(t==len)return 0;

  return s[l+t]<=s[nowl+t];

}

bool check(){

  int i,j,k=0;

  for(i=n-1;~i;i=j,k++){

    for(j=i;~j;j--)if(!ask(j,i))break;

    if(j==i)return 0;

  }

  return k<=K;

}

int main(){

  scanf("%d%s",&K,s);

  n=strlen(s);

  suffixarray(n,128);

  for(i=2;i<=n;i++)Log[i]=Log[i>>1]+1;

  for(i=1;i<=n;i++)f[0][i]=height[i];

  for(j=1;j<17;j++)for(i=1;i+(1<<j-1)<=n;i++)f[j][i]=min(f[j-1][i],f[j-1][i+(1<<j-1)]);

  for(i=1;i<=n;i++)R+=n-sa[i]-height[i];

  while(L<=R){

    kth(mid=(L+R)>>1);

    if(check())ansl=nowl,ansr=nowr,R=mid-1;else L=mid+1;

  }

  for(i=ansl;i<=ansr;i++)putchar(s[i]);

  return 0;

}