//By SiriusRen
#include <cstdio>
#include <cstring>
#include <algorithm>
using namespace std;
const int N=100050;
typedef long long ll;
int n,k,cntA[N],cntB[N],A[N],B[N],sa[N],tsa[N],rk[N],ht[N],Log[N],st[N][20];
int L,R;ll l,r,ans;
char s[N];
void SA(){for(int i=1;i<=n;i++)cntA[s[i]]++;for(int i=1;i<=256;i++)cntA[i]+=cntA[i-1];for(int i=n;i;i--)sa[cntA[s[i]]--]=i;rk[sa[1]]=1;for(int i=2;i<=n;i++)rk[sa[i]]=rk[sa[i-1]]+(s[sa[i]]!=s[sa[i-1]]);for(int l=1;rk[sa[n]]<n;l<<=1){memset(cntA,0,sizeof(cntA));memset(cntB,0,sizeof(cntB));for(int i=1;i<=n;i++)cntA[A[i]=rk[i]]++,cntB[B[i]=i+l<=n?rk[i+l]:0]++;for(int i=1;i<=n;i++)cntA[i]+=cntA[i-1],cntB[i]+=cntB[i-1];for(int i=n;i;i--)tsa[cntB[B[i]]--]=i;for(int i=n;i;i--)sa[cntA[A[tsa[i]]]--]=tsa[i];rk[sa[1]]=1;for(int i=2;i<=n;i++)rk[sa[i]]=rk[sa[i-1]]+(A[sa[i]]!=A[sa[i-1]]||B[sa[i]]!=B[sa[i-1]]);}for(int i=1,j=0;i<=n;i++){j=j?j-1:0,Log[i]=i!=1?Log[i>>1]+1:0;while(s[i+j]==s[sa[rk[i]-1]+j])j++;st[rk[i]][0]=ht[rk[i]]=j;}for(int j=1;j<=19;j++)for(int i=1;i+(1<<(j-1))<=n;i++)st[i][j]=min(st[i][j-1],st[i+(1<<(j-1))][j-1]);
}
int lcp(int x,int y){if(x==y)return n-x+1;x=rk[x],y=rk[y];if(x>y)swap(x,y);x++;int t=Log[y-x+1];return min(st[x][t],st[y-(1<<t)+1][t]);
}
void get(ll k){for(int i=1;i<=n;i++){if(n-sa[i]-ht[i]+1<k)k=k-(n-sa[i]-ht[i]+1);else{L=sa[i],R=sa[i]+ht[i]+k-1;break;}}
}
bool cmp(int l1,int r1,int l2,int r2){int len1=r1-l1+1,len2=r2-l2+1,LCP=lcp(l1,l2);if(len1<=len2&&LCP>=len1)return 1;if(len1>len2&&LCP>=len2)return 0;if(LCP>=len1&&LCP>=len2)return len1<len2;return s[l1+LCP]<s[l2+LCP];
}
bool check(){int cnt=1,last=n;for(int i=n;i;i--){if(!cmp(i,last,L,R))cnt++,last=i;if(cnt>k)return 0;}return 1;
}
int main(){scanf("%d%s",&k,s+1),n=strlen(s+1),SA();for(int i=1;i<=n;i++)r+=n-sa[i]-ht[i]+1;while(l<=r){ll mid=(l+r)>>1;get(mid);if(check())r=mid-1,ans=mid;else l=mid+1;}get(ans);s[R+1]=0;printf("%s\n",s+L);
}