#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 long long ll;
const int N=30;
ll dp[N][N][2];
string g[N];
ll ways(int n,int m,string s){
L(i,0,N-1)L(j,0,N-1)L(k,0,2)dp[i][j][k]=0;
dp[0][0][0]=1;
L(i,0,n-1){
L(j,0,m-1){
if(i>0){
dp[i][j][1]+=dp[i-1][j][1];
if(g[i][j]<=s[i+j])dp[i][j][g[i][j]!=s[i+j]]+=dp[i-1][j][0];
}
if(j>0){
dp[i][j][1]+=dp[i][j-1][1];
if(g[i][j]<=s[i+j])dp[i][j][g[i][j]!=s[i+j]]+=dp[i][j-1][0];
}
}
}
return dp[n-1][m-1][0]+dp[n-1][m-1][1];
}
int main(){
ios::sync_with_stdio(false);
cin.tie(nullptr);
int n,m;cin>>n>>m;
L(i,0,n-1)cin>>g[i];
ll k;cin>>k;
string ans(1,g[0][0]);
L(i,1,n+m-2){
for(char c='a';c<='z';c++){
string s=ans+c;
s.resize(n+m-1,'z');
if(ways(n,m,s)>=k){
ans+=c;
break;
}
}
}
cout<<ans<<endl;
}
