using System;
using static System.Console;
using System.Linq;
using System.Collections.Generic;

class Program
{
    static int NN => int.Parse(ReadLine());
    static int[] NList => ReadLine().Split().Select(int.Parse).ToArray();
    static string[] SList(int n) => Enumerable.Repeat(0, n).Select(_ => ReadLine()).ToArray();
    static void Main()
    {
        var c = NList;
        var (h, w) = (c[0], c[1]);
        var s = SList(h);
        
        var res = new List<char>();
        res.Add(s[0][0]);
        var prevmin = s[0][0];
        for (var t = 0; t < h + w - 2; ++t)
        {
            var next = new List<(int x, int y)>();
            var min = '{';
            for (var y = Math.Min(t, w - 1); y >= 0; --y)
            {
                var x = t - y;
                if (x >= h) break;
                if (s[x][y] == prevmin)
                {
                    if (x + 1 < h) min = (char)Math.Min(min, s[x + 1][y]);
                    if (y + 1 < w) min = (char)Math.Min(min, s[x][y + 1]);
                }
            }
            res.Add(min);
            prevmin = min;
        }
        WriteLine(string.Concat(res));
    }
}