#include <bits/stdc++.h>
using namespace std;
#define int long long
#define FASTIO ios_base::sync_with_stdio(false); cin.tie(NULL); cout.tie(NULL);
#define all(x) (x).begin(), (x).end()
#define pb push_back
#define pii pair<int, int>
#define ff first
#define ss second
#define PI acos(-1)
#define ld long double
template <class T> bool ckmin(T& a, const T& b) {return a > b ? a = b, true : false;}
template <class T> bool ckmax(T& a, const T& b) {return a < b ? a = b, true : false;}
const int mod = 1e9+7, N = 35;
int msb(int val){return sizeof(int)*8-__builtin_clzll(val)-1;}
int n, m, k;
char a[N][N];
vector<string> v;
string tmp;
void dfs(int i, int j){
tmp.pb(a[i][j]);
if(i == n-1 && j == m-1){
v.pb(tmp);
tmp.pop_back();
return;
}
if(i+1 < n)dfs(i+1,j);
if(j+1 < m)dfs(i,j+1);
tmp.pop_back();
}
void solve(int test_case){
int i, j;
cin >> n >> m;
for(i=0;i<n;i++){
for(j=0;j<m;j++){
cin >> a[i][j];
}
}
dfs(0, 0);
sort(all(v));
cin >> k;
cout << v[k-1] << '\n';
return;
}
signed main(){
FASTIO;
#define MULTITEST 0
#if MULTITEST
int _T;
cin >> _T;
for(int T_CASE = 1; T_CASE <= _T; T_CASE++)
solve(T_CASE);
#else
solve(1);
#endif
return 0;
}
