#include <bits/stdc++.h>
using namespace std;
int n, m;
vector < vector < char > > grid;
void bfs() {
queue < pair < pair < int, int >, int > > q;
vector < vector < int > > visited(n, vector < int >(m, 0));
q.push(make_pair(make_pair(0, 0), 0));
vector < char > min_char(n + m - 1, 'z');
min_char[0] = grid[0][0];
visited[0][0] = 1;
while (!q.empty()) {
int i = q.front().first.first;
int j = q.front().first.second;
int d = q.front().second;
q.pop();
if (grid[i][j] != min_char[d]) continue;
if (i < n - 1) {
if (!visited[i + 1][j]) {
q.push(make_pair(make_pair(i + 1, j), d + 1));
min_char[d + 1] = min(min_char[d + 1], grid[i + 1][j]);
visited[i + 1][j] = 1;
}
}
if (j < m - 1) {
if (!visited[i][j + 1]) {
q.push(make_pair(make_pair(i, j + 1), d + 1));
min_char[d + 1] = min(min_char[d + 1], grid[i][j + 1]);
visited[i][j + 1] = 1;
}
}
}
for (int i = 0; i < n + m - 1; ++i) {
cout << min_char[i];
}
cout << '\n';
}
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];
}
}
bfs();
return 0;
}
