#include <cassert>
#include <cmath>
#include <algorithm>
#include <iostream>
#include <iomanip>
#include <climits>
#include <map>
#include <queue>
#include <set>
#include <cstring>
#include <vector>

using namespace std;
typedef long long ll;

struct Node {
  int y;
  int x;
  char ch;

  Node(int y = -1, int x = -1, char ch = -1) {
    this->y = y;
    this->x = x;
    this->ch = ch;
  }

  bool operator>(const Node &n) const {
    return ch > n.ch;
  }
};

int main() {
  int H, W;
  cin >> H >> W;

  vector<string> S(H);
  for (int y = 0; y < H; ++y) {
    cin >> S[y];
  }

  queue<Node> que;
  que.push(Node(0, 0, S[0][0]));

  int history[H][W];
  memset(history, -1, sizeof(history));
  string ans = "";
  ans += S[0][0];
  bool visited[H][W];
  memset(visited, false, sizeof(visited));

  for (int i = 0; i < H + W - 2; ++i) {
    priority_queue <Node, vector<Node>, greater<Node>> pque;

    while (not que.empty()) {
      Node node = que.front();
      que.pop();

      if (node.y + 1 < H) {
        pque.push(Node(node.y + 1, node.x, S[node.y + 1][node.x]));
      }
      if (node.x + 1 < W) {
        pque.push(Node(node.y, node.x + 1, S[node.y][node.x + 1]));
      }
    }

    char ch = pque.top().ch;
    ans += ch;

    while (not pque.empty()) {
      Node node = pque.top();
      pque.pop();

      if (node.ch > ch) break;
      if (visited[node.y][node.x]) continue;
      visited[node.y][node.x] = true;

      que.push(node);
    }
  }

  cout << ans << endl;

  return 0;
}