#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 

int main(){
  constexpr const ll A = 3000;
  CIN( ll , H );
  CIN( ll , W );
  string S[A];
  FOR( i , 0 , H ){
    cin >> S[i];
  }
  string s{};
  ll i = 0;
  ll j = 0;
  ll H_m = H - 1;
  ll W_m = W - 1;
  ll N = H + W_m;
  FOR( n , 0 , N ){
    cout << S[i].substr( j , 1 );
    if( i == H_m ){
      j++;
    } else if( j == W_m ){
      i++;
    } else {
      auto ci = S[i+1].substr( j , 1 ).c_str()[0];
      auto cj = S[i].substr( j+1 , 1 ).c_str()[0];
      if( ci < cj ){
	i++;
      } else {
	j++;
      }
    }
  }
  cout << S[i].substr( j , 1 );
  RETURN( "" );
}