#include <bits/stdc++.h>
using namespace std;
int n;
int m;
vector<vector<char>> board;
int main(){
cin >> n >> m;
board = vector<vector<char>>(n,vector<char>(m));
queue<pair<int,int>> q;
string ans="";
for(int i=0;i<n;i++){
for(int j=0;j<m;j++){
cin >> board[i][j];
}
}
q.push({0,0});
ans+=board[0][0];
for(int diagonal=0;diagonal<(m+n-2);diagonal++){
char best='{';
set<pair<int,int>> coords;
while(!q.empty()){
pair<int,int> p = q.front();
q.pop();
int i=p.first;
int j=p.second;
if(i!=n-1){
if(board[i+1][j]<=best){
best=board[i+1][j];
coords.insert({i+1,j});
}
}
if(j!=m-1){
if(board[i][j+1]<=best){
best=board[i][j+1];
coords.insert({i,j+1});
}
}
}
ans+=best;
for(auto coord:coords){
if (board[coord.first][coord.second]==best){
q.push(coord);
}
}
}
cout << ans << endl;
}
