#include <iostream>
#include <vector>
#include <map>
#include <algorithm>
#include <cmath>
#include <iomanip>
#include <string>
#include <list>
#include <cassert>
#include <numeric>
#include <cstdint>
#include <queue>
#include <deque>
#include <stack>
#include <set>
#include <random>
#include <bitset>
// #include <atcoder/all>
 
using ll = long long;
using ld = long double;
using namespace std;
// using namespace atcoder;
using P = pair<ll, ll>;
using Graph = vector<vector<int>>;
using Priority = priority_queue<ll, vector<ll>, greater<ll>>;// 昇順
using Priority_pair = priority_queue<P, vector<P>, greater<P>>;
// using mint_17 = modint1000000007;
// using mint = modint998244353;

#define mod 1000000007
#define MAX_WIDTH 60
#define MAX_HEIGHT 60
#define INF 1e18
#define MOD 998244353
#define PI 3.141592653589793
#define rep(i, a, b) for(ll i=(a);i<(b);i++)
#define rrep(i, a, b) for(ll i=(a);i>=(b);i--)
#define fore(i, a) for(auto &i: a)
#define all(v) (v).begin(), (v).end()
#define YES(a) cout << ((a) ? "YES" : "NO") << endl
#define Yes(a) cout << ((a) ? "Yes" : "No") << endl
#define pb(a) push_back(a)
#define vi vector<int>
#define vll vector<ll>
#define vs vector<string>
#define vc vector<char>
#define vp vector<pair<ll, ll>>
#define mll map<ll, ll>
#define msl map<string, ll>
#define COUT(n) cout << (n) << endl
#define isInGrid(a, b, h, w) (0 <= (a) && (a) < (h) && 0 <= (b) && (b) < (w))
template<typename T> inline bool chmin(T& a, T b) { if (a > b) { a = b; return true; } return false; }
template<typename T> inline bool chmax(T& a, T b) { if (a < b) { a = b; return true; } return false; }
int vx[] = {1, 0, -1, 0}, vy[] = {0, 1, 0, -1};
struct edge {
    ll to, cost;
};

ll H, W, G[3010][3010];
char C[3010][3010];
int main() {
    cin >> H >> W;
    rep(i,0,H) {
        rep(j,0,W) {
            cin >> C[i][j];
            ll n = C[i][j]-'a'+1;
            G[i][j] = n;
        }
    }

    string ans;
    queue<pair<ll, ll>> que;
    ll dist[3010][3010];
    dist[0][0] = 0;
    que.push({0, 0});
    while(!que.empty()) {
        pair<ll, ll> v = que.front();
        que.pop();
        ll h = v.first, w = v.second;

        ll a = INF, b = INF;
        if(h+1 < H) {
            a = G[h+1][w];
        }

        if(w+1 < W) {
            b = G[h][w+1];
        }

        if(a == INF && b == INF) {
            //pass
        } else if(a == INF) {
            dist[h][w+1] = dist[h][w]+1;
            que.push(make_pair(h, w+1));
        } else if(b == INF) {
            dist[h+1][w] = dist[h][w]+1;
            que.push(make_pair(h+1, w));
        } else {
            if(a == b) {
                dist[h+1][w] = dist[h][w]+1;
                que.push(make_pair(h+1, w));

                dist[h][w+1] = dist[h][w]+1;
                que.push(make_pair(h, w+1));
            } else if(a < b) {
                dist[h+1][w] = dist[h][w]+1;
                que.push(make_pair(h+1, w));
            } else {
                dist[h][w+1] = dist[h][w]+1;
                que.push(make_pair(h, w+1));
            }
        }
    }

    ll now_h = H-1, now_w = W-1;
    while(1) {
        ans = ans + C[now_h][now_w];
        if(now_h == H-1 && now_w == W-1) {
            break;
        }

        if(now_h == 0) {
            now_w--;
        } else  if(now_w == 0) {
            now_h--;
        } else {
            if(dist[now_h-1][now_w] < dist[now_h][now_w-1]) {
                now_h--;
            } else {
                now_w--;
            }
        }
    }
    reverse(all(ans));
    COUT(ans);

    return 0;
}