#include <iostream>
#include <algorithm>
#include <iomanip>
#include <vector>
#include <queue>
#include <deque>
#include <set>
#include <map>
#include <tuple>
#include <cmath>
#include <numeric>
#include <functional>
#include <cassert>

#define debug_value(x) cerr << "line" << __LINE__ << ":<" << __func__ << ">:" << #x << "=" << x << endl;
#define debug(x) cerr << "line" << __LINE__ << ":<" << __func__ << ">:" << x << endl;

template<class T> inline bool chmax(T& a, T b) { if (a < b) { a = b; return 1; } return 0; }
template<class T> inline bool chmin(T& a, T b) { if (a > b) { a = b; return 1; } return 0; }

using namespace std;
typedef long long ll;

template<typename T>
vector<vector<T>> vec2d(int n, int m, T v){
    return vector<vector<T>>(n, vector<T>(m, v));
}

template<typename T>
vector<vector<vector<T>>> vec3d(int n, int m, int k, T v){
    return vector<vector<vector<T>>>(n, vector<vector<T>>(m, vector<T>(k, v)));
}

template<typename T>
void print_vector(vector<T> v, char delimiter=' '){
    if(v.empty()) {
        cout << endl;
        return;
    }
    for(int i = 0; i+1 < v.size(); i++) cout << v[i] << delimiter;
    cout << v.back() << endl;
}

int main(){
    ios::sync_with_stdio(false);
    cin.tie(0);
    cout << setprecision(10) << fixed;
    int h, w; cin >> h >> w;
    vector<string> s(h);
    for(int i = 0; i < h; i++) cin >> s[i];
    auto ok = vec2d(h, w, false);
    ok[0][0] = true;
    string ans;
    for(int x = 0; x <= h+w-2; x++){
        char mn = 'z';
        char cur = 'z';
        for(int j = 0; j <= x && j < w; j++){
            int i = x-j;
            if(i >= h) continue;
            if(!ok[i][j]) continue;
            cur = s[i][j];
            if(i+1 < h) chmin(mn, s[i+1][j]);
            if(j+1 < w) chmin(mn, s[i][j+1]);
        }
        ans += cur;
        for(int j = 0; j <= x && j < w; j++){
            int i = x-j;
            if(i >= h) continue;
            if(!ok[i][j]) continue;
            cur = s[i][j];
            if(i+1 < h && s[i+1][j] == mn) ok[i+1][j] = true;
            if(j+1 < w && s[i][j+1] == mn) ok[i][j+1] = true;
        }
    }
    cout << ans << endl;
}