#include<cstdio>
#include<cstring>
#include<algorithm>using namespace std;#define N 100001typedef long long LL;const int base=13331;int n,m;
char s[N];int a[N];
int v[N];
int p,q=1,k;
int sa[2][N],rk[2][N];
int h[N];unsigned long long Pow[N],has[N];pair<int,int>interval[N];void mul(int *sa,int*rk,int *SA,int *RK)
{for(int i=1;i<=n;++i) v[rk[sa[i]]]=i;for(int i=n;i;--i) if(sa[i]>k) SA[v[rk[sa[i]-k]]--]=sa[i]-k;for(int i=n-k+1;i<=n;++i) SA[v[rk[i]]--]=i;for(int i=1;i<=n;++i) RK[SA[i]]=RK[SA[i-1]]+(rk[SA[i]]!=rk[SA[i-1]] || rk[SA[i]+k]!=rk[SA[i-1]+k]);
}void presa()
{for(int i=1;i<=n;++i) v[a[i]]++;for(int i=1;i<=26;++i) v[i]+=v[i-1];for(int i=1;i<=n;++i) sa[p][v[a[i]]--]=i;for(int i=1;i<=n;++i) rk[p][sa[p][i]]=rk[p][sa[p][i-1]]+(a[sa[p][i]]!=a[sa[p][i-1]]);for(k=1;k<n;k<<=1,swap(p,q)) mul(sa[p],rk[p],sa[q],rk[q]);
}void get_height()
{int j,k=0;for(int i=1;i<=n;++i){j=sa[p][rk[p][i]-1];while(a[i+k]==a[j+k]) k++;h[rk[p][i]]=k;if(k) k--;}
}void prehash()
{Pow[0]=1;for(int i=1;i<=n;++i) Pow[i]=Pow[i-1]*base;for(int i=1;i<=n;++i) has[i]=has[i-1]*base+a[i];
}pair<int,int>select(LL k)
{int now;LL sum=0;int l,r;for(int i=1;i<=n;++i) {now=interval[i].second-interval[i].first+1;if(sum+now>=k){l=sa[p][i];r=interval[i].first+k-sum-1;return make_pair(l,r);}sum+=now;}
}unsigned long long get_hash(int l,int r)
{return has[r]-has[l-1]*Pow[r-l+1];
}int cmp(pair<int,int>x,pair<int,int>y)
{if(get_hash(x.first,x.second)==get_hash(y.first,y.second)) return 0;int Lx=x.second-x.first+1,Ly=y.second-y.first+1;int l=1,r=min(Lx,Ly),mid,tmp=0;while(l<=r){mid=l+r>>1;if(get_hash(x.first,x.first+mid-1)==get_hash(y.first,y.first+mid-1)) tmp=mid,l=mid+1;else r=mid-1;}if(tmp<min(Lx,Ly)) return s[x.first+tmp]<s[y.first+tmp] ? -1 : 1;return Lx<Ly ? -1 : 1;
}bool check(pair<int,int>now)
{int l=n,r=n,sum=1;while(l>=1)if(cmp(make_pair(l,r),now)==1) {if(l==r) return false;r=l;sum++;if(sum>m) return false;}else l--;return true;
}void solve()
{LL l=1,r=0;for(int i=1;i<=n;++i){interval[i].first=sa[p][i]+h[i];interval[i].second=n;r+=interval[i].second-interval[i].first+1;}LL mid,tmp;pair<int,int>now;while(l<=r){mid=l+r>>1;now=select(mid);if(check(now)) tmp=mid,r=mid-1;else l=mid+1;}now=select(tmp);l=now.first; r=now.second;for(int i=l;i<=r;++i) putchar(s[i]);
}    int main()
{freopen("string.in","r",stdin);freopen("string.out","w",stdout);scanf("%d",&m);scanf("%s",s+1);n=strlen(s+1);for(int i=1;i<=n;++i) a[i]=s[i]-'a'+1;presa();get_height();prehash();solve();
}