/*

This Submission is to determine how many 120/240 min const. delivery point there are.

//info
120 req. steps <= 5
*/
#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 1e18
#define GOLD 1.61803398874989484820458
#define MAX_MOD 1000000007LL
#define MOD 998244353LL
#define seg_size 262144
#define REP(a,b) for(long long a = 0;a < b;++a)

unsigned long xor128() {
	static unsigned long x = time(NULL), y = 362436069, z = 521288629, w = 88675123;
	unsigned long t = (x ^ (x << 11));
	x = y; y = z; z = w;
	return (w = (w ^ (w >> 19)) ^ (t ^ (t >> 8)));
}
double dot(complex<double> a, complex<double> b) {
	return a.real() * b.real() + a.imag() * b.imag();
}
double gyaku_dot(complex<double> a, complex<double> b) {
	return a.real() * b.imag() - a.imag() * b.real();
}
double leng(complex<double> a) {
	return sqrt(a.real() * a.real() + a.imag() * a.imag());
}
double angles(complex<double> a, complex<double> b) {
	double cosine = dot(a, b) / (leng(a) * leng(b));
	double sine = gyaku_dot(a, b) / (leng(a) * leng(b));
	double kaku = acos(min((double)1.0, max((double)-1.0, cosine)));
	if (sine <= 0) {
		kaku = 2 * Ma_PI - kaku;
	}
	return kaku;
}
vector<int> convex_hull(vector<complex<double>> a) {
	vector<int> ans;
	double now_minnest = a[0].real();
	int now_itr = 0;
	REP(i, a.size()) {
		if (now_minnest > a[i].real()) {
			now_minnest = a[i].real();
			now_itr = i;
		}
	}
	ans.push_back(now_itr);
	complex<double> ba(0, 1);
	while (true) {
		int now_go = 0;
		double now_min = 0;
		double now_length = 0;
		int starter = ans[ans.size() - 1];
		for (int i = 0; i < a.size(); ++i) {
			if (i != starter) {
				double goa = angles(ba, a[i] - a[starter]);
				if (goa - now_min >= eps || (abs(goa - now_min) <= eps && (abs(a[i] - a[starter]) - now_length) >= eps)) {
					now_min = goa;
					now_go = i;
					now_length = abs(a[i] - a[starter]);
				}
			}
		}
		if (now_go == ans[0]) break;
		ans.push_back(now_go);
		ba = complex<double>(a[now_go] - a[starter]);
	}
	return ans;
}
vector<vector<pair<int, int>>> listings;
int h, w;
int visited[20];
vector<pair<int, int>> nowing;
void init(int now) {
	if (now == ((h+1)/2)*2) {
		listings.push_back(nowing);
		return;
	}
	if (visited[now] == 1) {
		init(now + 1);
		return;
	}
	visited[now] = 1;
	for (int i = now + 1; i < ((h + 1) / 2) * 2; ++i) {
		if (visited[i] == 1) continue;
		visited[i] = 1;
		nowing.push_back(make_pair(now, i));
		init(now + 1);
		nowing.pop_back();
		visited[i] = 0;
	}
	visited[now] = 0;
	return;
}
int main() {
	cin >> h >> w;
	init(0);
	vector<string> A;
	REP(i, h) {
		string s;
		cin >> s;
		A.push_back(s);
	}
	int bobo = 12;
	for (int i = 0; i < h; ++i) {
		for (int que = 0; que < listings.size(); ++que) {
			vector<string> inputs = A;
			if (h % 2 == 1) {
				inputs.push_back(inputs[i]);
			}
			vector<string> spliter[2];
			for (int j = 0; j < listings[que].size(); ++j) {
				spliter[0].push_back(inputs[listings[que][j].first]);
				spliter[1].push_back(inputs[listings[que][j].second]);
			}
			reverse(spliter[1].begin(), spliter[1].end());
			inputs.clear();
			REP(j, 2) {
				REP(t, spliter[j].size()) {
					inputs.push_back(spliter[j][t]);
				}
			}
			int visited[12] = {};
			int cnt = 0;
			for (int q = 0; q < w; ++q) {
				if (visited[q] == 1) continue;
				for (int j = q + 1; j < w; ++j) {
					if (visited[j] == 1) continue;
					int ok = 1;
					for (int t = 0; t < (h+1)/2*2; ++t) {
						if (inputs[t][q]!= inputs[(h + 1) / 2 * 2 - t - 1][j]) {
							ok = 0;
							break;
						}
					}
					if (ok == 1) {
						visited[q] = 1;
						visited[j] = 1;
						break;
					}
				}
				if (visited[q] == 0) {
					visited[q] = 1;
					cnt++;
					for (int j = 0; j < (h+1)/2*2; ++j) {
						if (inputs[j][q] != inputs[(h + 1) / 2 * 2 - j - 1][q]) {
							cnt = 10;
						}
					}
				}
			}
			if (cnt <= w % 2) {
				cout << "YES" << endl;
				return 0;
			}
			bobo = min(bobo, cnt);
		}
		if (h % 2 == 0) break;
	}
	cout << "NO" << endl;
	return 0;
}

Measured memory_bytes: 1212