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);
        WriteLine(Small(h, w, s));
    }
    static string Small(int h, int w, string[] s)
    {
        var res = new List<char>();
        res.Add(s[0][0]);
        var q = new List<(int x, int y)>();
        q.Add((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 = '{';
            foreach (var pos in q)
            {
                if (pos.x + 1 < h) min = (char)Math.Min(min, s[pos.x + 1][pos.y]);
                if (pos.y + 1 < w) min = (char)Math.Min(min, s[pos.x][pos.y + 1]);
            }
            foreach (var pos in q)
            {
                if (pos.y + 1 < w && s[pos.x][pos.y + 1] == min)
                {
                    if (next.Count == 0 || next[next.Count - 1].x != pos.x)
                    {
                        next.Add((pos.x, pos.y + 1));
                    }
                }
                if (pos.x + 1 < h && s[pos.x + 1][pos.y] == min)
                {
                    next.Add((pos.x + 1, pos.y));
                }
            }
            res.Add(min);
            q = next;
        }
        return (string.Concat(res));
    }
}