/* Ajay Jadhav */
#include <iostream>
#include <cstdio>
#include <algorithm>
#include <deque>
#include <vector>
#include <cstdlib>
#include <iomanip>
#include <cmath>
#include <queue>
#include <map>
#include <set>
#include <stack>
#include <ctime>
#include <string.h>
#include <climits>
#include <cstring>
using namespace std;
#define ll long long
#define pb push_back
#define pii pair<int, int>
#define vi vector<int>
#define vvi vector<vi>
#define vii vector<pii>
#define mi map<int, int>
#define mii map<pii, int>
#define all(a) (a).begin(), (a).end()
#define x first
#define y second
#define sz(x) (int)x.size()
#define hell 1000000007
#define rep(i, a, b) for (int i = a; i < b; i++)
#define endl '\n'
string ans;
void trace_direction(vector<string> &g, vector<vector<char>> &dir, int i, int j, string tmp)
{
if (i < 0 or j < 0)
{
reverse(all(tmp));
if (ans == "")
ans = tmp;
else
ans = min(ans, tmp);
return;
}
if (dir[i][j] == 'b')
{
trace_direction(g, dir, i - 1, j, tmp + g[i][j]);
trace_direction(g, dir, i, j - 1, tmp + g[i][j]);
}
else if (dir[i][j] == 'l')
{
trace_direction(g, dir, i, j - 1, tmp + g[i][j]);
}
else
{
trace_direction(g, dir, i - 1, j, tmp + g[i][j]);
}
}
void solve()
{
int n, m;
cin >> n >> m;
vector<string> g(n);
rep(i, 0, n)
{
cin >> g[i];
}
vvi dp(n, vi(m, 0));
vector<vector<char>> dir(n, vector<char>(m, 'x'));
dp[0][0] = (g[0][0] - 'a');
dir[0][0] = 'u';
rep(i, 1, n)
{
dp[i][0] = dp[i - 1][0] + (g[i][0] - 'a');
dir[i][0] = 'u';
}
rep(i, 1, m)
{
dp[0][i] = dp[0][i - 1] + (g[0][i] - 'a');
dir[0][i] = 'l';
}
rep(i, 1, n)
{
rep(j, 1, m)
{
if (dp[i - 1][j] == dp[i][j - 1])
{
dp[i][j] += dp[i - 1][j] + (g[i][j] - 'a');
dir[i][j] = 'b';
}
else if (dp[i - 1][j] < dp[i][j - 1])
{
dp[i][j] += dp[i - 1][j] + (g[i][j] - 'a');
dir[i][j] = 'u';
}
else
{
dp[i][j] += dp[i][j - 1] + (g[i][j] - 'a');
dir[i][j] = 'l';
}
}
}
int i = n - 1, j = m - 1;
trace_direction(g, dir, n - 1, m - 1, "");
cout << ans << endl;
}
signed main()
{
ios::sync_with_stdio(false);
cin.tie(0);
cout.tie(0);
int t = 1;
// cin>>t;
while (t--)
{
solve();
}
return 0;
}
