#include <iostream>
#include <string>
#include <vector>
#include <algorithm>
#include <functional>
#include <cmath>
#include <iomanip>
#include <stack>
#include <queue>
#include <numeric>
#include <map>
#include <unordered_map>
#include <set>
#include <fstream>
#include <chrono>
#include <random>
#include <bitset>
//#include <atcoder/all>
#include <sstream>
#define rep(i,n) for(int i=0;i<(n);i++)
#define all(x) x.begin(), x.end()
#define rall(x) x.rbegin(), x.rend()
#define sz(x) ((int)(x).size())
#define pb push_back
using ll = long long;
using namespace std;
template<class T>bool chmax(T &a, const T &b) { if (a<b) { a=b; return 1; } return 0; }
template<class T>bool chmin(T &a, const T &b) { if (b<a) { a=b; return 1; } return 0; }
ll gcd(ll a, ll b) {return b?gcd(b,a%b):a;}
ll lcm(ll a, ll b) {return a/gcd(a,b)*b;}

int main(){
  int H,W; cin >> H >> W;
  vector<string> A(H); rep(i,H) cin >> A[i];
  string ans = "";
  ans += A[0][0];
  set<pair<int,int>> pos;
  pos.insert({0,0});
  rep(i,H+W-2){
    map<char,set<pair<int,int>>> mp;
    for(auto p:pos){
      int y=p.first,x=p.second;
      if(y+1<H){
        mp[A[y+1][x]].insert({y+1,x});
      }
      if(x+1<W){
        mp[A[y][x+1]].insert({y,x+1});
      }
    }
    char ad = (*mp.begin()).first;
    ans += ad;
    pos = (*mp.begin()).second;
  }
  cout << ans << endl;
  return 0;
}