import std.stdio, std.conv, std.functional, std.string;
import std.algorithm, std.array, std.container, std.range, std.typecons;
import std.bigint, std.numeric, std.math, std.random;
import core.bitop;

string FMT_F = "%.10f";

static File _f;
void file_io(string fn) { _f = File(fn, "r"); }
static string[] s_rd;
T _RD(T = long)() { while(!s_rd.length) s_rd = readln.chomp.split; string res = s_rd[0]; s_rd.popFront; return res.to!T; }
T _RD(T = long)(File f) { while(!s_rd.length) s_rd = f.readln.chomp.split; string res = s_rd[0]; s_rd.popFront; return res.to!T; }
T[] _RDA(T = long)(T fix = 0) { auto r = readln.chomp.split.to!(T[]); r[] += fix; return r; }
T[] _RDA(T = long)(File f, T fix = 0) { auto r = f.readln.chomp.split.to!(T[]); r[] += fix; return r; }
T RD(T = long)() { if (_f.isOpen) return _RD!T(_f); else return _RD!T; }
T[] RDA(T = long)(T fix = 0) { if (_f.isOpen)  return _RDA!T(_f, fix); else return _RDA!T(fix); }

size_t[] MAKE_IDX(alias less = "a < b", Range)(Range range) { auto idx = new size_t[](range.length); makeIndex!(less)(range, idx); return idx;}
size_t MIN_POS(alias less = "a < b", Range)(Range range) { auto r = minPos!(less)(range); return range.length - r.length; }

void chmin(T)(ref T x, T y) { x = min(x, y); } void chmax(T)(ref T x, T y) { x = max(x, y); }
bool inside(T)(T x, T b, T e) { return x >= b && x < e; }
T lcm(T)(T x, T y) { return x * (y / gcd(x, y)); }
T euDist2(T)(T[] a, T[] b) { auto c = a.dup; c[] -= b[]; c[] *= c[]; return c.sum; }
double euDist(T)(T[] a, T[] b) { return sqrt(cast(double)euDist2(a,b)); }
double[] rotate(double[] vec, double rad) { return [cos(rad)*vec[0] - sin(rad)*vec[1], sin(rad)*vec[0] + cos(rad)*vec[1]]; }
double norm(double[] vec) { return sqrt(reduce!((a,b)=>a+b*b)(0.0, vec)); }
double dotProd(double[] a, double[] b) { auto r = a.dup; r[] *= b[]; return r.sum; }

//long mod = 10^^9 + 7;
long mod = 998_244_353;
//long mod = 1_000_003;
void moda(ref long x, long y) { x = (x + y) % mod; }
void mods(ref long x, long y) { x = ((x + mod) - (y % mod)) % mod; }
void modm(ref long x, long y) { x = (x * y) % mod; }
void modpow(ref long x, long y) { if (!y) { x = 1; return; } auto t = x; x.modpow(y>>1); x.modm(x); if (y&1) x.modm(t); }
void modd(ref long x, long y) { y.modpow(mod - 2); x.modm(y); }

void main()
{
	auto H = RD;
	auto W = RD;
	auto S = new string[](H);
	foreach (i; 0..H)
		S[i] = RD!string;

	string ans = [S[0][0]];
	long[][] q = [[0, 0]];
	while (ans.length != H+W-1)
	{
		char c = cast(char)('z'+1);
		long[][] nq;
		foreach (nd; q)
		{
			auto i = nd[0];
			auto j = nd[1];
			if (i != H-1)
			{
				if (S[i+1][j] < c)
				{
					nq.length = 0;
					c = S[i+1][j];
				}
				if (S[i+1][j] == c)
					nq ~= [i+1, j];
			}
			if (j != W-1)
			{
				if (S[i][j+1] < c)
				{
					nq.length = 0;
					c = S[i][j+1];
				}
				if (S[i][j+1] == c)
					nq ~= [i, j+1];
			}
		}
		q = nq;
		ans ~= c;
	}

	writeln(ans);
	stdout.flush;
	debug readln;
}