#include <bits/stdc++.h>
#define int long long
#define fi first
#define se second
#define sz(a) (int)((a).size())
#define all(a) (a).begin(), (a).end()
#define lsb(x) (x & (-x))
#define vi vector<int>
#define YES { cout << "Yes" << endl; return; }
#define NO { cout << "No" << endl; return; }
using ll = long long;
using pii = std::pair<int, int>;
const int NMAX = 30;
using namespace std;
char a[NMAX + 5][NMAX + 5];
int dp[NMAX + 5][NMAX + 5][2], n, m, k;
int solve(string& sol) {
fill(&dp[0][0][0], &dp[0][0][0] + (NMAX + 5) * (NMAX + 5) * 2, 0LL);
dp[1][1][0] = 1;
for (int i = 1; i <= n; ++i) {
for (int j = 1; j <= m; ++j) {
for (int lower = 0; lower < 2; ++lower) {
if (i + 1 <= n) {
int new_lower = (lower | (a[i + 1][j] < sol[i + j - 1]));
if (!(new_lower == 0 && a[i + 1][j] > sol[i + j - 1]))
dp[i + 1][j][new_lower] += dp[i][j][lower];
}
if (j + 1 <= m) {
int new_lower = (lower | (a[i][j + 1] < sol[i + j - 1]));
if (!(new_lower == 0 && a[i][j + 1] > sol[i + j - 1]))
dp[i][j + 1][new_lower] += dp[i][j][lower];
}
}
}
}
return dp[n][m][0] + dp[n][m][1];
}
signed main() {
cin.tie(nullptr)->sync_with_stdio(false);
cin >> n >> m;
for (int i = 1; i <= n; ++i)
for (int j = 1; j <= m; ++j)
cin >> a[i][j];
cin >> k;
string ans(n + m - 1, 'z');
ans[0] = a[1][1];
for (int i = 2; i <= n + m - 1; ++i) {
int lo = 0, hi = 25, best = 0;
while (lo <= hi) {
auto mid = (lo + hi) >> 1;
ans[i - 1] = static_cast<char>(mid + 'a');
if (solve(ans) >= k) {
best = mid;
hi = mid - 1;
}
else
lo = mid + 1;
}
ans[i - 1] = static_cast<char>(best + 'a');
}
cout << ans << '\n';
return 0;
}
