#include <bits/stdc++.h>
using namespace std;
using P = pair<int, int>;
int main () {
    int H, W;
    cin >> H >> W;
    char A[3030][3030];
    for (int i = 0; i < H; i ++) {
        for (int j = 0; j < W; j ++) {
            cin >> A[i][j];
        }
    }
    for (int i = 0; i < H; i ++) {
        A[i][W] = '{';
    }
    for (int j = 0; j < W; j ++) {
        A[H][j] = '{';
    }
    vector<P> now = {P{0, 0}};
    cout << A[0][0];
    string key = "abcdefghijklmnopqrstuvwxyz";
    for (int i = 2; i < H + W; i ++) {
        std::vector<P> X[50];
        for (auto [x, y] : now) {
            X[A[x + 1][y] - 'a'].emplace_back(x + 1, y);
            X[A[x][y + 1] - 'a'].emplace_back(x, y + 1);
        }
        int fl = 0;
        while (X[fl].empty()) {
            fl ++;
        }
        cout << key[fl];
        now.clear();
        sort(X[fl].begin(), X[fl].end());
        auto result = std::unique(X[fl].begin(), X[fl].end());
        X[fl].erase(result, X[fl].end());
        for (auto x : X[fl]) {
            now.push_back(x);
        }
    }
    cout << endl;
}