#include <cstdio>
#include <cstring>
#include <algorithm>
using namespace std ;
int len , K , sa[5005] , rk[5005] , he[5005] ;
int tp[5005] , buc[5005] ;
char ss[5005] ;
bool cmp( int *rk , int x , int y , int d ){
return rk[x] != rk[y] || ( rk[x] == rk[y] && rk[x+d] != rk[y+d] ) ;
}
void Rsort( int *tp , int *rk , int M ){
for( int i = 0 ; i <= M ; i ++ ) buc[i] = 0 ;
for( int i = 1 ; i <= len ; i ++ ) buc[ rk[i] ] ++ ;
for( int i = 1 ; i <= M ; i ++ ) buc[i] += buc[i-1] ;
for( int i = len ; i ; i -- ) sa[ buc[rk[tp[i]]] -- ] = tp[i] ;
}
void getSA(){
int *tp = ::tp , *rk = ::rk , m = 127 ;
for( int i = 1 ; i <= len ; i ++ ) tp[i] = i , rk[i] = ss[i] ;
Rsort( tp , rk , m ) ;
for( int l = 1 , p = 0 ; ; l <<= 1 , p = 0 ){
for( int i = len - l + 1 ; i <= len ; i ++ ) tp[++p] = i ;
for( int i = 1 ; i <= len ; i ++ )
if( sa[i] > l ) tp[++p] = sa[i] - l ;
Rsort( tp , rk , m ) , swap( tp , rk ) , rk[ sa[1] ] = 1 ;
for( int i = 2 ; i <= len ; i ++ )
rk[ sa[i] ] = rk[ sa[i-1] ] + cmp( tp , sa[i] , sa[i-1] , l ) ;
if( rk[ sa[len] ] == len ) break ;
m = rk[ sa[len] ] ;
} if( rk != ::rk ) memcpy( ::rk , rk , sizeof( rk ) ) ;
for( int i = 1 , j , k = 0 ; i <= len ; he[rk[i]] = k , i ++ )
for( k=k?k-1:k , j = sa[rk[i]-1] ; ss[i+k] == ss[j+k] ; k ++ ) ;
}
pair<int,int> Query( int K ){
for( int i = 1 ; i <= len ; i ++ ){
if( len - sa[i] - he[i] + 1 >= K )
return make_pair( sa[i] , sa[i] + he[i] + K - 1 ) ;
else K -= len - sa[i] - he[i] + 1 ;
}
}
void solve(){
len = strlen( ss + 1 ) ;
getSA() ;
pair<int,int> intval = Query( K ) ;
for( int i = intval.first ; i <= intval.second ; i ++ )
printf( "%c" , ss[i] ) ;
}
int main(){
scanf( "%s%d" , ss + 1 , &K ) ;
solve() ;
}