#include <iostream>
#include <vector>
#include <string>
#include <algorithm>
using namespace std;
using P = pair<int, int>;

int main(){
    int h, w; cin >> h >> w;
    vector<string> s(h);
    for(auto &it: s) cin >> it;

    vector<vector<vector<P>>> cnt(h+w, vector<vector<P>>(26));
    cnt[0][s[0][0]-'a'].emplace_back(0, 0);
    vector<vector<P>> pre(h, vector<P>(w, {-1, -1}));
    for(int phase = 0; phase < h+w-1; phase++){
        for(int x = 0; x < 26; x++){
            if(cnt[phase][x].size() == 0) continue;
            for(auto &it: cnt[phase][x]){
                int i = it.first, j = it.second;
                // cerr << i << " " << j << ", ";
                if(i+1 < h){
                    cnt[phase+1][s[i+1][j]-'a'].emplace_back(i+1, j);
                    pre[i+1][j] = {i, j};
                }
                if(j+1 < w){
                    cnt[phase+1][s[i][j+1]-'a'].emplace_back(i, j+1);
                    pre[i][j+1] = {i, j};
                }
            }
            // cerr << endl;
            break;
        }
    }

    string ans = "";
    P cur = {h-1, w-1};
    while(cur != P{-1, -1}){
        ans += s[cur.first][cur.second];
        cur = pre[cur.first][cur.second];
    }
    reverse(ans.begin(), ans.end());
    cout << ans << endl;
    return 0;
}