#define  _CRT_SECURE_NO_WARNINGS
#include <stdio.h>
#include <algorithm>
#include <utility>
#include <functional>
#include <cstring>
#include <queue>
#include <stack>
#include <math.h>
#include <iterator>
#include <vector>
#include <string>
#include <set>
#include <math.h>
#include <iostream>
#include <random>
#include<map>
#include <iomanip>
#include <time.h>
#include <stdlib.h>
#include <list>
#include <typeinfo>
#include <list>
#include <set>
#include <cassert>
#include<fstream>
#include <unordered_map>
#include <cstdlib>
#include <complex>
#include <cctype>
#include <bitset>
using namespace std;
typedef string::const_iterator State;
#define Ma_PI 3.141592653589793
#define eps 1e-5
#define LONG_INF 2000000000000000000LL
#define GOLD 1.61803398874989484820458
#define MAX_MOD 1000000007LL
#define GYAKU 500000004LL
#define MOD 998244353LL
#define seg_size 262144
#define REP(a,b) for(long long a = 0;a < b;++a)
int main() {
	int h, w;
	cin >> h >> w;
	vector<string> inputs;
	REP(i, h) {
		string s;
		cin >> s;
		inputs.push_back(s);
	}
	int ok = 1;
	for (int i = 0; i < h - 1; ++i) {
		int tea = 0;
		for (int q = 0; q < w; ++q) {
			if (inputs[i][q] != inputs[i + 1][q]) tea = 1;
		}
		ok &= tea;
	}
	for (int i = 0; i < w - 1; ++i) {
		int tea = 0;
		for (int q = 0; q < h; ++q) {
			if (inputs[q][i] != inputs[q][i+1]) tea = 1;
		}
		ok &= tea;
	}
	string s[2] = { "NO","YES" };
	cout << s[ok] << endl;
}