#include <iostream>
#include <vector>
#include <algorithm>
#include <map>
#include <queue>
#include <set>
#include <random>
using namespace std;
typedef long long ll;
#define rep(i, n) for(int i = 0; i < (n); i++)

template<class T>
using vi = vector<T>;

template<class T>
using vii = vector<vi<T>>;

template<class T>
using viii = vector<vii<T>>;

using pii = pair<int, int>;

struct status {
    int x, y;
};

int main()
{
    int h, w;
    cin >> h >> w;
    vi<string> s(h);
    rep(i, h) cin >> s[i];

    queue<status> q;
    q.push({ 0, 0 });
    char c = s[0][0];
    cout << c;

    vii<bool> visited(h, vi<bool>(w));
    const int dx[2] = { 1, 0 };
    const int dy[2] = { 0, 1 };
    for (int sm = 0; sm < h + w - 2; sm++) {
        char nc = 'z';
        while (!q.empty() && q.front().x + q.front().y == sm) {
            status v = q.front();
            q.pop();
            if (s[v.x][v.y] != c) continue;
            if (visited[v.x][v.y]) continue;
            visited[v.x][v.y] = true;
            rep(i, 2) {
                int nx = v.x + dx[i];
                int ny = v.y + dy[i];
                if (nx >= h || ny >= w) continue;
                if (s[nx][ny] <= nc) {
                    q.push({ nx, ny });
                    nc = s[nx][ny];
                }
            }
        }
        c = nc;
        cout << c;
    }
    cout << endl;

    return 0;
}