#include <iostream>
#include <vector>
using namespace std;
typedef pair<int, int> point;
#define x first
#define y second
const int MAX = 2010;
int n, m;
char grid[MAX][MAX];
bool vis[MAX][MAX];
int main(void) {
cin >> n >> m;
for (int i = 0; i < n; ++i)
cin >> grid[i];
vector< point > curr, next;
for (curr.push_back({0, 0}); !curr.empty(); curr = next) {
point p = curr.back();
cout << grid[p.x][p.y];
char mn = 'z';
for (point pt : curr) {
int dx = 1, dy = 0;
for (int i = 0; i < 2; ++i) {
swap(dx, dy);
int nx = pt.x + dx;
int ny = pt.y + dy;
if (nx >= n || ny >= m) continue;
mn = min(mn, grid[nx][ny]);
}
}
next.clear();
for (point pt : curr) {
int dx = 1, dy = 0;
for (int i = 0; i < 2; ++i) {
swap(dx, dy);
int nx = pt.x + dx;
int ny = pt.y + dy;
if (nx >= n || ny >= m) continue;
if (vis[nx][ny]) continue;
if (grid[nx][ny] == mn) {
next.push_back({nx, ny});
vis[nx][ny] = 1;
}
}
}
}
cout << endl;
return 0;
}
