/* -*- coding: utf-8 -*-
 *
 * 2064.cc:  No.2064 Smallest Sequence on Grid - yukicoder
 */

#include<cstdio>
#include<algorithm>
 
using namespace std;

/* constant */

const int MAX_H = 3000;
const int MAX_W = 3000;
const int MAX_L = MAX_H + MAX_W - 1;

/* typedef */

/* global variables */

char fs[MAX_H][MAX_W + 4], t[MAX_L + 4];
bool used[MAX_H][MAX_W];

/* subroutines */

/* main */

int main() {
  int h, w;
  scanf("%d%d", &h, &w);
  for (int i = 0; i < h; i++) scanf("%s", fs[i]);

  int l = h + w - 1;
  used[0][0] = true;

  for (int k = 0; k < l; k++) {
    int i0 = min(k, h - 1), j0 = k - i0;
    char minc = 'z' + 1;
    for (int i = i0, j = j0; i >= 0 && j < w; i--, j++)
      if (used[i][j]) minc = min(minc, fs[i][j]);
    t[k] = minc;

    for (int i = i0, j = j0; i >= 0 && j < w; i--, j++)
      if (used[i][j] && fs[i][j] == minc) {
	if (i + 1 < h) used[i + 1][j] = true;
	if (j + 1 < w) used[i][j + 1] = true;
      }
  }

  puts(t);
  return 0;
}