#include <iostream>
#include <vector>
#include <algorithm>
using namespace std;
const int N = 35;
#define int long long
int dp[N][N], n, m, k, Dp[N][N];
char a[N][N];
void get(vector<pair<char, pair<int, int>>> v){
// cout<<"here again!!!!!!!!!!!!!!!!!!"<<endl;
cout<<v.back().first;
if (v[0].second == make_pair(n, m))
return;
vector<pair<char, pair<int, int>>> v2;
for (auto [c, pr] : v){
auto [i, j] = pr;
if (i + 1 <= n)
v2.push_back({a[i+1][j], {i+1, j}});
if (j + 1 <= m)
v2.push_back({a[i][j+1], {i, j+1}});
}
sort(begin(v2), end(v2));
v2.resize(unique(begin(v2), end(v2)) - begin(v2));
int k2 = 0;
char prv = '0';
for (auto [c, pr] : v2){
auto [i, j] = pr;
if (c != prv and k2 < k){
k -= k2, k2 = 0;
prv = c;
}
Dp[i][j] = Dp[i-1][j] + Dp[i][j-1];
k2 += dp[i][j] * Dp[i][j];
// cout<<i<<" "<<j<<" "<<k2<<" "<<k<<endl;
if (k2 >= k){
// cout<<"done"<<endl;
while (v2.size() > 0 and v2.back().first != c){
auto [i1, j1] = v2.back().second;
Dp[i1][j1] = 0;
v2.pop_back();
}
while (v2.size() > 0 and v2.front().first != c){
auto [i1, j1] = v2.front().second;
Dp[i1][j1] = 0;
v2.erase(begin(v2));
}
break;
}
}
get(v2);
}
signed main(){
cin>>n>>m;
for (int i=1;i<=n;i++){
for (int j=1;j<=m;j++)
cin>>a[i][j];
}
dp[n][m] = 1;
for (int i=n;i>=1;i--){
for (int j=m;j>=1;j--)
dp[i][j] += dp[i+1][j] + dp[i][j+1];
}
// for (int i=1;i<=n;i++){
// for (int j=1;j<=m;j++){
// if (dp[i][j] < 10)
// cout<<' ';
// cout<<dp[i][j]<<' ';
// }
// cout<<'\n';
// }
cin>>k;
Dp[1][1] = 1;
get({{a[1][1], {1, 1}}});
}
