#include<iostream>

#include<cstdio>

#include<cmath>

#include<cstdlib>

#include<cstring>

#include<algorithm>

using namespace std;

int read()

{

    int x=,f=;char c=getchar();

    while (c<''||c>'') {if (c=='-') f=-;c=getchar();}

    while (c>=''&&c<='') x=(x<<)+(x<<)+(c^),c=getchar();

    return x*f;

}

#define N 100010

#define ll long long

int n,m,sa[N],sa2[N],rk[N<<],tmp[N<<],h[N],cnt[N],l[N];

char s[N];

ll tot=;

void make()

{

    int m=;

    for (int i=;i<=n;i++) cnt[rk[i]=s[i]]++,m=max(m,(int)s[i]);

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

    for (int i=n;i>=;i--) sa[cnt[rk[i]]--]=i;

    for (int k=;k<=n;k<<=)

    {

        int p=;

        for (int i=n-k+;i<=n;i++) sa2[++p]=i;

        for (int i=;i<=n;i++) if (sa[i]>k) sa2[++p]=sa[i]-k;

        memset(cnt,,sizeof(cnt));

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

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

        for (int i=n;i>=;i--) sa[cnt[rk[sa2[i]]]--]=sa2[i];

        memcpy(tmp,rk,sizeof(rk));

        p=rk[sa[]]=;

        for (int i=;i<=n;i++)

        {

            if (tmp[sa[i]]!=tmp[sa[i-]]||tmp[sa[i]+k]!=tmp[sa[i-]+k]) p++;

            rk[sa[i]]=p;

        }

        if (p==n) break;

        m=p;

    }

    for (int i=;i<=n;i++)

    {

        h[i]=max(h[i-]-,);

        while (s[i+h[i]]==s[sa[rk[i]-]+h[i]]) h[i]++;

    }

}

bool check(ll k)

{

    int x=;

    for (int i=;i<=n;i++)

    if (k<=n-sa[i]+-h[sa[i]]) {x=i;break;}

    else k-=n-sa[i]+-h[sa[i]];

    k+=h[sa[x]];memset(l,,sizeof(l));

    int lcp=k;if (k<n-sa[x]+) l[sa[x]+lcp-]=max(l[sa[x]+lcp-],sa[x]);

    for (int i=x+;i<=n;i++)

    {

        lcp=min(lcp,h[sa[i]]);

        if (!lcp) return ;

        l[sa[i]+lcp-]=max(l[sa[i]+lcp-],sa[i]);

    }

    int t=;

    for (int i=;i<=n;i++)

    if (l[i])

    {

        t++;if (t>m) return ;

        int x=i;while (x<n&&l[x+]<=i) x++;

        i=x;

    }

    return ;

}

int main()

{

#ifndef ONLINE_JUDGE

    freopen("bzoj4310.in","r",stdin);

    freopen("bzoj4310.out","w",stdout);

    const char LL[]="%I64d\n";

#else

    const char LL[]="%lld\n";

#endif

    m=read();

    scanf("%s",s+);n=strlen(s+);

    make();

    tot=1ll*n*(n+)>>;

    for (int i=;i<=n;i++) tot-=h[i];

    ll l=,r=tot,ans;

    while (l<=r)

    {

        ll mid=l+r>>;

        if (check(mid)) r=mid-,ans=mid;

        else l=mid+;

    }

    for (int i=;i<=n;i++)

    if (ans<=n-sa[i]+-h[sa[i]]) {for (int j=sa[i];ans+h[sa[i]]>;j++,ans--)printf("%c",s[j]);break;}

    else ans-=n-sa[i]+-h[sa[i]];

    return ;

}