#include <iostream>
#include <vector>
#include <string>
using namespace std;

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

    string ans = "";
    int i = 0, j = 0;
    while(true){
        ans += s[i][j];
        if(i == h-1 && j == w-1) break;
        if(i+1 < h && j+1 < w){
            if(s[i+1][j] < s[i][j+1]) i++;
            else j++;
        }else if(i+1 < h) i++;
        else j++;
    }
    cout << ans << endl;
    return 0;
}