#include <bits/stdc++.h>
#include <ext/pb_ds/assoc_container.hpp>
#include <ext/pb_ds/tree_policy.hpp>
using namespace std;
using namespace __gnu_pbds;
#define int long long
#define ordered_set tree<int, null_type, less<int>, rb_tree_tag, tree_order_statistics_node_update>
#define ordered_multiset tree<int, null_type, less_equal<int>, rb_tree_tag, tree_order_statistics_node_update>
#define intmin -100000000000000000LL
#define intmax 100000000000000000LL
vector<vector<int>> arr(2002, vector<int>(2002, -1));
vector<vector<string>> dp(2002, vector<string>(2002, "!"));
int n, m;
string recurse(int i, int j){
if(i > n || j > m){
return "~";
}
if(dp[i][j] != "!"){
return dp[i][j];
}
string ans = "";
ans += char(97+arr[i][j]);
if(i == n && j == m){
return dp[i][j] = ans;
}
ans += min(recurse(i+1, j), recurse(i, j+1));
return dp[i][j] = ans;
}
int32_t main(){
ios_base::sync_with_stdio(false);
cin.tie(NULL);
cin >> n >> m;
for(int i = 1; i <= n; i++){
for(int j = 1; j <= m; j++){
char c;
cin >> c;
arr[i][j] = c - 'a';
}
}
cout << recurse(1, 1) << '\n';
}
