#pragma GCC target("avx2")
#pragma GCC optimize("O3")
#pragma GCC optimize("unroll-loops")
#include <stdio.h>
#include <math.h>
#include <iostream>
#include <iomanip>
#include <vector>
#include <set>
#include <map>
#include <unordered_set>
#include <unordered_map>
#include <queue>
#include <string>
#include <stack>
#include <numeric>
#include <algorithm>
using namespace std;
using ll = long long;
#include <atcoder/all>
using namespace atcoder;
using mint = modint998244353;
#define rep(i, n) for (ll i = 0; i < n; i++)
#define ALL(a) (a).begin(), (a).end()
void chmin(ll &a, ll b)
{
    if (a > b)
        a = b;
}
void chmax(ll &a, ll b)
{
    if (a < b)
        a = b;
}
int main()
{
    cin.tie(nullptr);
    ios::sync_with_stdio(false);
    int h, w;
    cin >> h >> w;
    vector<string> ss(h);
    for (int i = 0; i < h; i++)
    {
        cin >> ss[i];
    }
    vector<vector<int>> s(h, vector<int>(w));
    for (int i = 0; i < h; i++)
        for (int j = 0; j < w; j++)
            s[i][j] = ss[i][j] - 'a';
    vector<vector<int>> yu(h, vector<int>(w)), ha(h, vector<int>(w));
    const int inf = 5e7;
    vector<int> size(h + w - 1);
    for (int i = 0; i < h; i++)
        for (int j = 0; j < w; j++)
            size[i + j]++;
    for (int i, j, ij = h + w - 3; ij >= 0; ij--)
    {
        int ii = 0, jj = ij;
        if (jj >= w)
        {
            int sa = jj - w + 1;
            ii += sa;
            jj -= sa;
        }
        i = ii;
        j = jj;
        vector<int> hash(size[ij]);
        int count = 0;
        while (i < h && j >= 0)
        {
            hash[count++] = ha[i][j] = s[i][j] * inf + min(i == h - 1 ? inf : yu[i + 1][j], j == w - 1 ? inf : yu[i][j + 1]);
            i++;
            j--;
        }
        i = ii;
        j = jj;
        sort(ALL(hash));
        while (i < h && j >= 0)
        {
            int ok = 0, ng = size[ij];
            while (ng - ok != 1)
            {
                int vs = (ok + ng) >> 1;
                if (hash[vs] <= ha[i][j])
                    ok = vs;
                else
                    ng = vs;
            }
            yu[i][j] = ok;
            i++;
            j--;
        }
    }
    int i = 0, j = 0;
    while (j != w)
    {
        cout << ss[i][j];
        if (i == h - 1)
            j++;
        else if (j == w - 1)
            i++;
        else
        {
            int hi = yu[i + 1][j], hj = yu[i][j + 1];
            if (hi < hj)
                i++;
            else
                j++;
        }
    }
    return 0;
}