#include <bits/stdc++.h>
using namespace std;
int n, m;
vector < vector < char > > grid;
vector < vector < int > > adj;
string s = "", ans = "";
void dfs(int u) {
int i = u / m, j = u % m;
s += grid[i][j];
if (u == n * m - 1) {
if (ans == "") ans = s;
else ans = min(ans, s);
} else {
if (i == n - 1) dfs(u + 1);
else if (j == m - 1) dfs(u + 1);
else {
if (grid[i + 1][j] <= grid[i][j + 1]) dfs(u + m);
if (grid[i + 1][j] >= grid[i][j + 1]) dfs(u + 1);
}
}
s.pop_back();
}
int main()
{
cin >> n >> m;
grid = vector < vector < char > >(n, vector < char >(m));
for (int i = 0; i < n; ++i) {
for (int j = 0; j < m; ++j) {
cin >> grid[i][j];
}
}
adj = vector < vector < int > >(n * m);
for (int i = 0; i < n; ++i) {
for (int j = 0; j < m; ++j) {
if (i < n - 1) adj[i * m + j].push_back((i + 1) * m + j);
if (j < m - 1) adj[i * m + j].push_back(i * m + j + 1);
}
}
dfs(0);
cout << ans << '\n';
return 0;
}
