#include<bits/stdc++.h>
using namespace std;

#pragma GCC optimize("O3")

#define rep(i,n) for(ll i=0;i<n;i++)
#define repl(i,l,r) for(ll i=(l);i<(r);i++)
#define per(i,n) for(ll i=(n)-1;i>=0;i--)
#define perl(i,r,l) for(ll i=r-1;i>=l;i--)
#define fi first
#define se second
#define pb push_back
#define ins insert
#define pqueue(x) priority_queue<x,vector<x>,greater<x>>
#define all(x) (x).begin(),(x).end()
#define CST(x) cout<<fixed<<setprecision(x)
#define vtpl(x,y,z) vector<tuple<x,y,z>>
#define rev(x) reverse(x);
using ll=long long;
using vl=vector<ll>;
using vvl=vector<vector<ll>>;
using pl=pair<ll,ll>;
using vpl=vector<pl>;
using vvpl=vector<vpl>;
const ll MOD=1000000007;
const ll MOD9=998244353;
const int inf=1e9+10;
const ll INF=4e18;
const ll dy[9]={0,1,0,-1,1,1,-1,-1,0};
const ll dx[9]={1,0,-1,0,1,-1,1,-1,0};
template<class T> inline bool chmin(T& a, T b) {
    if (a > b) {
        a = b;
        return true;
    }
    return false;
}
template<class T> inline bool chmax(T& a, T b) {
    if (a < b) {
        a = b;
        return true;
    }
    return false;
}
int main(){
  ll h,w;cin >> h >> w;
  vector<string> g(h);rep(i,h)cin >> g[i];
  string ans;
  set<pl> st;st.insert({-1,0});
  rep(i,h+w-1){
    ll f=INF;
    for(auto [x,y]:st){
      rep(k,2){
        ll nx=x+dx[k],ny=y+dy[k];
        if(nx>=h||ny>=w||nx<0||ny<0)continue;
        chmin(f,ll(g[nx][ny]-'a'));
      }
    }
    ans+='a'+f;
    set<pl> nst;
    for(auto [x,y]:st){
      rep(k,2){
        ll nx=x+dx[k],ny=y+dy[k];
        if(nx>=h||ny>=w||nx<0||ny<0)continue;
        if(f==ll(g[nx][ny]-'a')){
          nst.insert(make_pair(nx,ny));
        }
      }
    }
    swap(st,nst);
  }
  cout << ans << endl;
}