#include<bits/stdc++.h>
using namespace std;
#define int long long
#define in pair<int, int>
#define f first
#define s second
#define pb push_back
#define pob pop_back
#define INF (int)1e17
#define MX (int)3e5+5
#define fast() ios_base::sync_with_stdio(false); cin.tie(NULL)
signed main()
{
fast();
int n, m;
cin >> n >> m;
char grid[n+1][m+1];
for(int i = 1; i <= n; i++)
{
for(int j = 1; j <= m; j++)
cin >> grid[i][j];
}
string curr = "";
curr.pb(grid[1][1]);
set<in> opt;
opt.insert({1,1});
int stages = n+m-2;
while(stages--)
{
vector<pair<in, char>> v;
char val = '{';
for(in ok: opt)
{
if(ok.f < n)
{
v.pb({{ok.f+1, ok.s}, grid[ok.f+1][ok.s]});
val = min(val, grid[ok.f+1][ok.s]);
}
if(ok.s < m)
{
v.pb({{ok.f, ok.s+1}, grid[ok.f][ok.s+1]});
val = min(val, grid[ok.f][ok.s+1]);
}
}
curr.pb(val);
opt.clear();
for(auto aaa: v)
{
if(aaa.s != val)
continue;
opt.insert(aaa.f);
}
}
cout << curr;
return 0;
}
