#include <iostream>
#include <fstream>

using namespace std;

ifstream fin("barcode.in");
ofstream fout("barcode.out");

int n, m;
char c[1000][1000];

void read()
{
	int i, j;

	fin >> n >> m;

	for (i = 0; i <= n; i++)
	{
		for (j = 0; j <= m; j++)
		{
			fin >> c[i][j];
		}
	}
}

int solve()
{
	int i, j, l = 0;
	char a;
	
	for (i = 0; i <= n; i++)
	{
		
		for (j = 1; j <= m; j++)
		{
			a = c[i - 1][j - 1];
			if (c[i][j] == a && l == 0) l = 1;
			else if (l && c[i][j] != a) l = 0;
		}
	}

	cout << l;
	return l;
}

int main()
{
	read();

	if (solve()) fout << "Yes" << endl;
	else fout << "No" << endl;


	system("pause");
	return 0;
}