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

vector<int> dx = {1, 0};
vector<int> dy = {0, 1};

int main() {
    ios::sync_with_stdio(0);
    cin.tie(0);
    int H, W;
    cin >> H >> W;
    vector<string> S(H);
    for (int i=0;i<H;i++) cin >> S[i];

    string ans = "";
    vector<pair<int, int>> vec;
    vec.push_back(make_pair(0, 0));
    while (true) {
        ans += S[vec[0].first][vec[0].second];
        vector<pair<int, int>> vec_nxt;
        char c_max = '{';
        for (auto ele : vec) {
            for (int i=0;i<2;i++) {
                int i_nxt = ele.first + dx[i];
                int j_nxt = ele.second + dy[i];
                if (!(0 <= i_nxt && i_nxt < H)) continue;
                if (!(0 <= j_nxt && j_nxt < W)) continue;
                if (S[i_nxt][j_nxt] < c_max) {
                    vec_nxt.clear();
                    c_max = S[i_nxt][j_nxt];
                    vec_nxt.push_back(make_pair(i_nxt, j_nxt));
                } else if (S[i_nxt][j_nxt] == c_max) {
                    vec_nxt.push_back(make_pair(i_nxt, j_nxt));
                }
            }
        }
        vec = vec_nxt;
        sort(vec.begin(), vec.end());
        vec.erase(unique(vec.begin(), vec.end()), vec.end());
        if (vec.size() == 0) break;
    }
    cout << ans << "\n";
    return 0;
}