#include <bits/stdc++.h>
#define ff first
#define ss second
#define pb push_back
#define mp make_pair
using namespace std;
typedef long long ll;
typedef unsigned long long ull;
typedef long double ld;
typedef pair<ll, ll> pll;
typedef pair<ull, ull> pull;
typedef pair<int, int> pii;
typedef pair<ld, ld> pld;
int n, m;
char a[2009][2009];
vector<pii> cur = {{1, 1}};
vector<pii> nxt;
string s;
int main() {
ios_base::sync_with_stdio(false);
cin.tie(nullptr);
//freopen("in.txt", "r", stdin);
//freopen("out.txt", "w", stdout);
cin >> n >> m;
for(int i = 1; i <= n; ++i)
for(int j = 1; j <= m; ++j)
cin >> a[i][j];
s.pb(a[1][1]);
while(1){
int mini = 1e9;
for(auto u : cur){
if(u == mp(n, m))
goto END;
if(u.ff+1 <= n)
mini = min((int)a[u.ff+1][u.ss], mini);
if(u.ss+1 <= m)
mini = min((int)a[u.ff][u.ss+1], mini);
}
s.pb(mini);
for(auto u : cur){
if(u.ff+1 <= n && a[u.ff+1][u.ss] == mini)
nxt.pb({u.ff+1, u.ss});
if(u.ss+1 <= m && a[u.ff][u.ss+1] == mini)
nxt.pb({u.ff, u.ss+1});
}
cur = nxt;
nxt.clear();
}
END:;
cout << s << "\n";
}
