#include <bits/stdc++.h>
using namespace std;
using ll=long long;
using ld=long double;
using P=pair<int,int>;
using V=vector<int>;
#define rep(i,n) for(int i=0;i<n;i++)
#define REP(i,n) for(int i=1;i<=n;i++)

int main(){
    int h,w;
    cin >> h >> w;
    vector<V> a(h,V(w));
    rep(i,h) rep(j,w){
        char c;
        cin >> c;
        a[i][j]=c-'a';
    }

    string t; t+=(char)('a'+a[0][0]);
    vector<set<P>> s(h+w-1);
    s[0].insert(P(0,0));
    rep(l,h+w-2){
        int c=100;
        for(auto i=s[l].begin();i!=s[l].end();++i){
            P p=*i;
            int x=p.first,y=p.second;
            if(x+1<h) c=min(c,a[x+1][y]);
            if(y+1<w) c=min(c,a[x][y+1]);
        }
        for(auto i=s[l].begin();i!=s[l].end();++i){
            P p=*i;
            int x=p.first,y=p.second;
            if(x+1<h) if(a[x+1][y]==c) s[l+1].insert(P(x+1,y));
            if(y+1<w) if(a[x][y+1]==c) s[l+1].insert(P(x,y+1));
        }
        t+=(char)('a'+c);
    }

    cout << t << endl;

}