#define _USE_MATH_DEFINES
#include <bits/stdc++.h>
using namespace std;
#define FOR(i,m,n) for(int i=(m);i<(n);++i)
#define REP(i,n) FOR(i,0,n)
#define ALL(v) (v).begin(),(v).end()
using ll = long long;
constexpr int INF = 0x3f3f3f3f;
constexpr long long LINF = 0x3f3f3f3f3f3f3f3fLL;
constexpr double EPS = 1e-8;
constexpr int MOD = 998244353;
// constexpr int MOD = 1000000007;
constexpr int DY4[]{1, 0, -1, 0}, DX4[]{0, -1, 0, 1};
constexpr int DY8[]{1, 1, 0, -1, -1, -1, 0, 1};
constexpr int DX8[]{0, -1, -1, -1, 0, 1, 1, 1};
template <typename T, typename U>
inline bool chmax(T& a, U b) { return a < b ? (a = b, true) : false; }
template <typename T, typename U>
inline bool chmin(T& a, U b) { return a > b ? (a = b, true) : false; }
struct IOSetup {
  IOSetup() {
    std::cin.tie(nullptr);
    std::ios_base::sync_with_stdio(false);
    std::cout << fixed << setprecision(20);
  }
} iosetup;

int main() {
  int h, w; cin >> h >> w;
  vector<string> s(h); REP(i, h) cin >> s[i];
  string ans(1, s[0][0]);
  set<pair<int, int>> st{{0, 0}};
  REP(_, h + w - 2) {
    set<pair<int, int>> nxt;
    char ch = '{';
    for (const auto [i, j] : st) {
      for (const int k : vector<int>{0, 3}) {
        const int y = i + DY4[k], x = j + DX4[k];
        if (0 <= y && y < h && 0 <= x && x < w) {
          if (chmin(ch, s[y][x])) {
            nxt.clear();
            nxt.emplace(y, x);
          } else if (ch == s[y][x]) {
            nxt.emplace(y, x);
          }
        }
      }
    }
    ans += ch;
    st.swap(nxt);
  }
  cout << ans << '\n';
  return 0;
}