#include <stdio.h>
#include <atcoder/all>
#include <bits/stdc++.h>
using namespace std;
using namespace atcoder;
using mint = modint998244353;
#define rep(i,n) for (int i = 0; i < (n); ++i)
#define Inf32 1000000001
#define Inf64 4000000000000000001

int main(){
	
	int H,W;
	cin>>H>>W;
	
	vector<string> S(H);
	rep(i,H)cin>>S[i];
	
	string ans = string(1,S[0][0]);
	vector<pair<int,int>> x(1,make_pair(0,0));
	
	rep(i,H+W-2){
		vector<pair<int,int>> nx;
		char c = 'z';
		rep(j,x.size()){
			rep(k,2){
				int xx = x[j].first,yy = x[j].second;
				if(k==0)xx++;
				else yy++;
				if(xx>=H||xx<0||yy>=W||yy<0)continue;
				c = min(c,S[xx][yy]);
			}
		}
		ans += c;
		rep(j,x.size()){
			rep(k,2){
				int xx = x[j].first,yy = x[j].second;
				if(k==0)xx++;
				else yy++;
				if(xx>=H||xx<0||yy>=W||yy<0)continue;
				if(S[xx][yy]==c){
					nx.emplace_back(xx,yy);
				}
			}
		}
		sort(nx.begin(),nx.end());
		nx.erase(unique(nx.begin(),nx.end()),nx.end());
		x = nx;
	}
	
	cout<<ans<<endl;
	
	
    return 0;
}