#pragma GCC optimize("Ofast")
#include <iostream>
#include <vector>
#include <algorithm>
#include <map>
#include <queue>
#include <cstdio>
#include <ctime>
#include <assert.h>
#include <chrono>
#include <random>
#include <numeric>
#include <set>
#include <deque>
#include <stack>
#include <sstream>
#include <utility>
#include <cstring>
#include <unordered_map>
#include <unordered_set>
#include <tuple>
#include <array>
#include <bitset>
using namespace std;
typedef long long int ll;
typedef unsigned long long ull;

mt19937_64 rng(chrono::steady_clock::now().time_since_epoch().count());
ll myRand(ll B) {
    return (ull)rng() % B;
}
inline double time() {
    return static_cast<long double>(chrono::duration_cast<chrono::nanoseconds>(chrono::steady_clock::now().time_since_epoch()).count()) * 1e-9;
}

int main(){
    cin.tie(nullptr);
    ios::sync_with_stdio(false);
    int h,w; cin >> h >> w;
    vector<string> s(h);
    for(int i=0;i<h;i++){
        cin >> s[i];
    }
    string res = "";
    res += s[0][0];
    vector<pair<int,int>> q;
    q.push_back({0,0});
    for(int i=1;i<h-1+w;i++){
        vector<pair<int,int>> nq;
        char c = 'z';
        for(auto p:q){
            int x = p.first, y = p.second;
            if(x+1<h){
                c = min(c, s[x+1][y]);
            }
            if(y+1<w){
                c = min(c, s[x][y+1]);
            }
        }
        res += c;
        for(auto p:q){
            int x = p.first, y = p.second;
            if(x+1<h and s[x+1][y] == c){
                nq.push_back({x+1,y});
            }
            if(y+1<w and s[x][y+1] == c){
                nq.push_back({x,y+1});
            }
        }
        swap(q,nq);
    }
    cout << res << endl;
}