#include<bits/stdc++.h>
namespace {
#pragma GCC diagnostic push
#pragma GCC diagnostic ignored "-Wunused-function"
#include<atcoder/all>
#pragma GCC diagnostic pop
using namespace std;
using namespace atcoder;
#define rep(i,n) for(int i = 0; i < (int)(n); i++)
#define rrep(i,n) for(int i = (int)(n) - 1; i >= 0; i--)
#define all(x) begin(x), end(x)
#define rall(x) rbegin(x), rend(x)
template<class T> bool chmax(T& a, const T& b) { if (a < b) { a = b; return true; } else return false; }
template<class T> bool chmin(T& a, const T& b) { if (b < a) { a = b; return true; } else return false; }
using ll = long long;
using P = pair<int,int>;
using VI = vector<int>;
using VVI = vector<VI>;
using VL = vector<ll>;
using VVL = vector<VL>;
const int di[] = {1, 0};
const int dj[] = {0, 1};

} int main() {
  ios::sync_with_stdio(false);
  cin.tie(0);
  int h, w;
  cin >> h >> w;
  vector<string> s(h);
  rep(i, h) cin >> s[i];
  string ans(1, s[0][0]);
  vector<P> pos{{0, 0}};
  auto idxchk = [&](int i, int j) { return 0 <= i && i < h && 0 <= j && j < w; };
  vector<vector<char>> visited(h, vector<char>(w));
  rep(_, h - 1 + w - 1) {
    vector<P> nxt;
    char mn = 'z' + 1;
    for(auto [i, j]: pos) {
      if (visited[i][j]) continue;
      visited[i][j] = true;
      rep(k, 2) {
        int ni = i + di[k], nj = j + dj[k];
        if (!idxchk(ni, nj)) continue;
        if (s[ni][nj] < mn) {
          mn = s[ni][nj];
          nxt.clear();
          nxt.emplace_back(ni, nj);
        } else if (s[ni][nj] == mn) {
          nxt.emplace_back(ni, nj);
        }
      }
    }
    ans += mn;
    swap(pos, nxt);
  }
  cout << ans << '\n';
}