#include <bits/stdc++.h>
#include <atcoder/all>
using namespace std;
using namespace atcoder;
using ll = long long;
using ld = long double;
using mint = modint998244353;

int main() {
  int H, W; cin >> H >> W;
  vector<string> S(H);
  for (int i = 0; i < H; i++) cin >> S[i];
  int dx[2] = {1, 0};
  int dy[2] = {0, 1};
  vector<vector<bool>> vis(H, vector<bool>(W, false));
  vector<pair<int, int>> A, B;
  A.emplace_back(0, 0);
  vis[0][0] = true;
  string ans;
  ans += S[0][0];
  for (int i = 0; i < H + W - 2; i++) {
    B.clear();
    char c = char('z' + 1);
    for (auto &p: A) {
      for (int j = 0; j < 2; j++) {
        int nx = p.first + dx[j];
        int ny = p.second + dy[j];
        if (nx >= H || ny >= W) continue;
        if (vis[nx][ny]) continue;
        if (c >= S[nx][ny]) {
          if (c > S[nx][ny]) B.clear();
          c = S[nx][ny];
          B.emplace_back(nx, ny);
          vis[nx][ny] = true;
        }
      }
    }
    ans += c;
    swap(A, B);
  }
  cout << ans << endl;
  return 0;
}