#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];
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]);
vector<pair<int,int>>path;
path.pb({0,0});
L(i,0,n+m-1){
bool vis[30][30];
memset(vis,0,sizeof(vis));
vector<pair<int,int>>nxtPath;
for(auto [x,y]:path){
if(x+1<n&&!vis[x+1][y]){
nxtPath.pb({x+1,y});
vis[x+1][y]=true;
}
if(y+1<m&&!vis[x][y+1]){
nxtPath.pb({x,y+1});
vis[x][y+1]=true;
}
}
for(char c='a';c<='z';c++){
ll ways=0;
for(auto [x,y]:nxtPath)if(g[x][y]==c){
ways+=dp[x][y];
}
if(ways>=k){
ans+=c;
path.clear();
for(auto [x,y]:nxtPath)if(g[x][y]==c){
path.pb({x,y});
}
break;
}else{
k-=ways;
}
}
}
cout<<ans<<endl;
}
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