#include <iostream>
#include <vector>
#include <algorithm>
#include <string>
#include <cmath>
#include <iomanip>
#include <bitset>
#include <functional>
#include <queue>
#include <stack>
#include <list>
#include <deque>
#include <utility>
#include <numeric>
#include <set>
#include <map>
#include <complex>
#include <cctype>
#include <climits>
#include <cassert>
#include <unordered_map>
#include <unordered_set>
#include <time.h>

using namespace std;

using ll = long long;
using ull = unsigned long long;
using ld = long double;
using uint = unsigned;
using vi = vector<int>;
using vl = vector<long>;
using vll = vector<long long>;
using vd = vector<double>;
using vvi = vector<vi>;
using vvl = vector<vl>;
using vvll = vector<vll>;
using vc = vector<char>;
using vs = vector<string>;
using vb = vector<bool>;
using pii = pair<int, int>;
using pcc = pair<char, char>;
using pll = pair<ll, ll>;
using pdd = pair<ld, ld>;
using vpii = vector<pii>;
using vpll = vector<pll>;

typedef complex<double> xy_t;
template<class T>bool chmax(T& a, const T& b) { if (a < b) { a = b; return 1; } return 0; }
template<class T>bool chmin(T& a, const T& b) { if (b < a) { a = b; return 1; } return 0; }

#define rep(i, n) for (ll i = 0; i < ll(n); i++)
#define repback(i, n) for (ll i = n-1; i >= 0; i--)
#define REP(i, a, b) for (ll i = a; i < ll(b); i++)
#define REPBACK(i, a, b) for (ll i = a-1; i >= ll(b); i--)
#define all(x) (x).begin(), (x).end()
#define EPS (1e-10)
#define equals(a,b) (fabs((a) - (b)) < EPS)
#define debug(x) cout << "debug:" << x << endl
#define debug2(x, y) cout << "debug:" << x << " " << y << endl

static const double pi = acos(-1.0);
const long long INFL = pow(10,18);
const long long INFLMAX = 9223372036854775807;
const int INF = pow(10,9);
const int INFMAX = 2147483647;
const int mod1 = 1000000007;
const int mod2 = 998244353;
const vi dx = {1,0,-1,0};
const vi dy = {0,1,0,-1};
const vi dx2 = {0, 1, 1, 1, 0, -1, -1, -1, 0};
const vi dy2 = {1, 1, 0, -1, -1, -1, 0, 1, 1};

int H = 3000;
int W = 3000;
vs S(H);

int main(){
    cin.tie(0);
    ios_base::sync_with_stdio(false);

    cin >> H >> W;

    rep(i,H) cin >> S[i];

    string ans = "";
    ans += S[0][0];
    vector<vb> able(H, vb(W, false));
    able[0][0] = true;
    //sumは起点の座標の和
    rep(sum,H+W-2){
        char now = 'z';
        rep(i,sum+1){
            int j = sum - i;
            if(i>=H || j<0 || j>=W) continue;
            //前回の点の候補でない
            if(!able[i][j]) continue;
            if(ans[sum] != S[i][j]) continue;

            if(i != H-1){
                chmin(now, S[i+1][j]);
                able[i+1][j] = true;
            }
            if(j != W-1){
                chmin(now, S[i][j+1]);
                able[i][j+1] = true;
            }
        }
        ans += now;
    }

    cout << ans << endl;

}