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

#define rep(i,n) for(int i = 0; i < (int)n; i++)
#define FOR(n) for(int i = 0; i < (int)n; i++)
#define repi(i,a,b) for(int i = (int)a; i < (int)b; i++)
#define pb push_back
#define m0(x) memset(x,0,sizeof(x))
#define fill(x,y) memset(x,y,sizeof(x))
#define bg begin()
#define ed end()
#define all(x) x.bg,x.ed
//#define mp make_pair
#define vi vector<int>
#define vvi vector<vi>
#define vll vector<ll>
#define vvll vector<vll>
#define vs vector<string>
#define vvs vector<vs>
#define vc vector<char>
#define vvc vector<vc>
#define pii pair<int,int>
#define pllll pair<ll,ll>
#define vpii vector<pair<int,int>>
#define vpllll vector<pair<ll,ll>>
#define vpis vector<pair<int,string>>
#define vplls vector<pair<ll, string>>
#define vpsi vector<pair<string, int>>
#define vpsll vector<pair<string, ll>>

template<typename T>
void chmax(T &a, const T &b) {a = (a > b? a : b);}
template<typename T>
void chmin(T &a, const T &b) {a = (a < b? a : b);}

using ll = long long;
using ld = long double;
using ull = unsigned long long;

const ll INF = numeric_limits<long long>::max() / 2;
const ld pi = 3.1415926535897932384626433832795028;
const ll mod = 1e9 + 7;
int dx[] = {-1, 0, 1, 0, -1, -1, 1, 1};
int dy[] = {0, -1, 0, 1, -1, 1, -1, 1};

#define int long long

void solve() {
    int h, w;
    cin >> h >> w;
    vector<string> s(h);
    FOR(h) cin >> s[i];

    pii init = {-1, -1};
    vector<vpii> pre(h, vpii(w, init));
    vvi done(h, vi(w, false));
    queue<pii> que;
    que.emplace(0, 0);
    
    while(que.size()) {
        vpii nxt;
        char c = s[que.front().first][que.front().second];

        while(que.size()) {
            int x, y;
            tie(x, y) = que.front();
            que.pop();

            if(c < s[x][y]) continue;

            vpii nnxt;

            if(x != h-1 && !done[x+1][y]) {
                pre[x+1][y] = {x, y};
                done[x+1][y] = true;
                nnxt.emplace_back(x+1, y);
            }
            if(y != w-1 && !done[x][y+1]) {
                pre[x][y+1] = {x, y};
                done[x][y+1] = true;
                nnxt.emplace_back(x, y+1);
            }

            sort(all(nnxt), [&](pii p, pii q) {
                return s[p.first][p.second] < s[q.first][q.second];
            });
            for(auto e : nnxt) nxt.push_back(e);
        }

        for(auto e : nxt) que.push(e);
    }

    int now_x = h-1, now_y = w-1;
    string ans = "";

    while(pre[now_x][now_y] != init) {
        ans.push_back(s[now_x][now_y]);
        tie(now_x, now_y) = pre[now_x][now_y];
    }
    ans.push_back(s[0][0]);

    reverse(all(ans));

    cout << ans << endl;

    // for(auto e : pre) {
    //     for(auto f : e) {
    //         cout << "{" << f.first << "," << f.second << "}" << " ";
    //     }
    //     cout << "\n";
    // }
}

signed main() {
    cin.tie(nullptr);
    ios::sync_with_stdio(false);
    solve();
    return 0;
}