#include <bits/stdc++.h>
#define L(i, j, k) for(int i = (j); i <= (k); i++)
#define R(i, j, k) for(int i = (j); i >= (k); i--)
#define all(x) x.begin(), x.end()
#define sz(a) ((int) a.size())
#define pb push_back
#define fst first
#define snd second
using namespace std;
typedef unsigned long long ll;
ll dp[30][30];
bool vis[30][30];
int main(){
ios::sync_with_stdio(false);
cin.tie(nullptr);
int n,m;cin>>n>>m;
vector<string>g(n);
L(i,0,n-1)cin>>g[i];
ll k;cin>>k;
dp[n-1][m-1]=1;
R(i,n-1,0){
R(j,m-1,0){
int ni=i-1,nj=j-1;
if(ni>=0)dp[ni][j]+=dp[i][j];
if(nj>=0)dp[i][nj]+=dp[i][j];
}
}
string ans(1,g[0][0]);
queue<pair<int,int>>q;
q.push({0,0});
vis[0][0]=true;
while(sz(q)){
auto [i,j]=q.front();q.pop();
if(i==n-1&&j==m-1)break;
vector<pair<char,pair<int,int>>>nxt;
if(i+1<n&&!vis[i+1][j]){
nxt.pb({g[i+1][j],{i+1,j}});
vis[i+1][j]=true;
}
if(j+1<m&&!vis[i][j+1]){
nxt.pb({g[i][j+1],{i,j+1}});
vis[i][j+1]=true;
}
sort(all(nxt));
ll ways=0;
for(auto [c,p]:nxt){
int ni=p.fst,nj=p.snd;
ways+=dp[ni][nj];
if(ways>=k){
q.push({ni,nj});
ans+=c;
break;
}else{
k-=ways;
}
}
}
cout<<ans<<endl;
}
