#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;
void Solve( string ( &S )[A] , ll& i , ll& j , string& s , const ll& H_m , const ll& W_m );

int main(){
  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;
  Solve( S , i , j , s , H_m , W_m );
  s += S[i].substr( j , 1 );
  RETURN( s );
}

void Solve( string ( &S )[A] , ll& i , ll& j , string& s , const ll& H_m , const ll& W_m )
{
  while( i != H_m || j != W_m ){
    s += 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 if( cj < ci ){
	j++;
      } else {
	string si{};
	string sj{};
	ll i_copy_i = i + 1;
	ll j_copy_i = j;
	ll i_copy_j = i;
	ll j_copy_j = j + 1;
	Solve( S , i_copy_i , j_copy_i , si , H_m , W_m );
	Solve( S , i_copy_j , j_copy_j , sj , H_m , W_m );
	if( si < sj ){
	  s += si;
	  i = i_copy_i;
	  j = j_copy_i;
	} else {
	  s += sj;
	  i = i_copy_j;
	  j = j_copy_j;
	}
      }
    }
  }
  return;
}