#include<cstdio>
#include<cstring>
#include<algorithm>

using namespace std;

#define N 100001

typedef 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();
}