#include<bits/stdc++.h>
using namespace std;

using ll = long long;

#define CIN( LL , A ) LL A; cin >> A 
#define FOR( VAR , INITIAL , FINAL_PLUS_ONE ) for( int VAR = INITIAL ; VAR < FINAL_PLUS_ONE ; VAR ++ ) 
#define RETURN( ANSWER ) cout << ( ANSWER ) << "\n"; return 0 

constexpr const ll A = 3000;

int main(){
  CIN( ll , H );
  CIN( ll , W );
  string S[A];
  FOR( i , 0 , H ){
    cin >> S[i];
  }
  vector<ll> i{};
  vector<ll> j{};
  vector<ll> i_prev{};
  vector<ll> j_prev{};
  i_prev.push_back( 0 );
  j_prev.push_back( 0 );
  cout << S[0].substr( 0 , 1 );
  ll H_m = H - 1;
  ll W_m = W - 1;
  ll L = H_m + W_m;
  FOR( l , 0 , L ){
    ll size = i_prev.size();
    string s_min = "z";
    FOR( k , 0 , size ){
      ll i_curr = i_prev[k] + 1;
      ll j_curr = j_prev[k];
      FOR( num , 0 , 2 ){
	if( i_curr <= H_m && j_curr <= W_m ){
	  string s_curr = S[i_curr].substr( j_curr , 1 );
	  if( s_min == s_curr ){
	    i.push_back( i_curr );
	    j.push_back( j_curr );
	  } else if( s_min > s_curr ){
	    s_min = s_curr;
	    i.clear();
	    j.clear();
	    i.push_back( i_curr );
	    j.push_back( j_curr );
	  }
	}
	i_curr--;
	j_curr++;
      }
    }
    cout << s_min;
    i_prev = i;
    j_prev = j;
    i.clear();
    j.clear();
  }
  RETURN( "" );
}