#define _CRT_SECURE_NO_WARNINGS
#define _SCL_SECURE_NO_WARNINGS
#include <cstdio>
#include <cstdlib>
#include <cstring>
#include <cassert>
#include <iostream>
#include <string>
#include <vector>
#include <list>
#include <utility>
#include <algorithm>
#include <functional>
#include <cmath>
#include <stack>
#include <queue>
#include <set>
#include <map>
#include <iomanip>
#include <sstream>
#include <bitset>
#include <limits>
#include <numeric>
#include <valarray>
#include <fstream>

using namespace std;
typedef unsigned int uint;
typedef long long LL;
typedef unsigned long long ULL;
typedef pair<LL, LL> PP;
#define REP(i, a, n) for(LL i = (a), i##_max = (n); i < i##_max; ++i)
#define REM(i, a, n) for(LL i = (LL)(n) - 1, i##min = (a); i >= i##min; --i)
#define ALL(arr) (arr).begin(), (arr).end()
#define FLOAT fixed << setprecision(16)
#define SPEEDUP {cin.tie(NULL); ios::sync_with_stdio(false);}
const int INF = 0x3FFFFFFF;
const LL INFLL = 0x3FFFFFFF3FFFFFFF;
const double INFD = 1.0e+308;
const string INFSTR = "\x7f";
const double EPS = 1.0e-9;

void YesNo(bool b) { cout << (b ? "Yes" : "No") << endl; }
void YESNO(bool b) { cout << (b ? "YES" : "NO") << endl; }
template <class T, class U>
istream& operator>>(istream& ist, pair<T, U>& right) { return ist >> right.first >> right.second; }
template <class T, class U>
ostream& operator<<(ostream& ost, const pair<T, U>& right) { return ost << right.first << ' ' << right.second; }
template <class T, class TCompatible, size_t N>
void Fill(T(&dest)[N], const TCompatible& val) { fill(dest, dest + N, val); }
template <class T, class TCompatible, size_t M, size_t N>
void Fill(T(&dest)[M][N], const TCompatible& val) { for (int i = 0; i < M; ++i) Fill(dest[i], val); }
template<class T>
T Compare(T left, T right) { return left > right ? 1 : (left < right ? -1 : 0); }
istream& Ignore(istream& ist) { string s; ist >> s; return ist; }
bool Inside(int i, int j, int h, int w) { return i >= 0 && i < h && j >= 0 && j < w; }
template <class T>
T Next() { T buf; cin >> buf; return buf; }

#ifdef ONLY_MY_ENVIR
#include "IntMod.h"
#include "Union_Find.h"
#include "Graph.h"
#include "Range.h"
#include "Global.h"
#include "Flow_Solver.h"
#include "Tree.h"
#include "Suffix_Array.h"
#include "Geometry.h"
#include "Matrix.h"
#include "Segment_Tree.h"
#include "Rational.h"
#include "Position.h"
#include "Factorization.h"
#endif

#ifdef __GNUC__
typedef __int128 LLL;
istream& operator>> (istream& ist, __int128& val) { LL tmp;  ist >> tmp; val = tmp; return ist; }
ostream& operator<< (ostream& ost, __int128 val) { LL tmp = val; ost << tmp; return ost; }
#endif

#if 1234567891
#include <array>
#include <random>
#include <unordered_set>
#include <unordered_map>
template<typename T>
using PriorityQ = priority_queue<T, vector<T>, greater<T> >;	// コスト小を優先
template <class T>
auto Is(const T& value) { return [value](const auto& comparand) -> bool { return comparand == value; }; }
#endif

using PC = pair<char, char>;

int H, W;
string s[12];
vector<vector<int>> pairs;

void Rec(int s, vector<int>& curr) {
	if (s == 0) {
		pairs.push_back(curr);
		return;
	}

	int v1, v2;
	REP(i, 0, W) {
		if ((s >> i) & 1) {
			v1 = i;
			s -= 1 << i;
			curr.push_back(v1);
			break;
		}
	}

	REP(i, 0, W) {
		if ((s >> i) & 1) {
			v2 = i;
			s -= 1 << i;
			curr.push_back(v2);
			Rec(s, curr);

			s += 1 << i;
			curr.pop_back();
		}
	}
	curr.pop_back();
	return;
}

void Pre() {
	if (W % 2 == 0) {
		vector<int> vec;
		Rec((1 << W) - 1, vec);
	} else {
		REP(i, 0, W) {
			vector<int> vec(2, i);
			Rec((1 << W) - 1 - (1 << i), vec);
		}
	}
}

uint pop_count(uint n) {
	n = (n & 0x55555555) + ((n >> 1) & 0x55555555);
	n = (n & 0x33333333) + ((n >> 2) & 0x33333333);
	n = (n & 0x0f0f0f0f) + ((n >> 4) & 0x0f0f0f0f);
	n = (n & 0x00ff00ff) + ((n >> 8) & 0x00ff00ff);
	n = (n & 0x0000ffff) + ((n >> 16) & 0x0000ffff);
	return n;
}

bool Sub(const vector<string>& ss) {
	bool used[12] = {};
	bool parin = false;
	REP(z, 0, (H + 1) / 2) {
		int p = 0;
		REP(i, 0, H) {
			if (used[i]) continue;
			p = i;
			break;
		}

		string t = ss[p];
		reverse(t.begin(), t.end());

		bool ok = false;
		REP(i, 0, H) {
			if (used[i]) continue;
			if (i == p) continue;
			if (ss[i] == t) {
				used[p] = true;
				used[i] = true;
				ok = true;
				break;
			}
		}
		if (!ok) {
			if (parin || H % 2 == 0 || t != ss[p]) {
				return false;
			}
			parin = true;
			used[p] = true;
		}
	}
	return true;
}

int main() {
	cin >> H >> W;
	REP(i, 0, H) {
		cin >> s[i];
	}

	Pre();

	REP(p, 0, pairs.size()) {
		vector<string> ss;
		REP(i, 0, H) {
			string buf;
			REP(j, 0, pairs[p].size()) {
				if (j % 2 == 0) {
					buf = s[i][pairs[p][j]] + buf;
				} else {
					buf += s[i][pairs[p][j]];
				}
			}
			ss.push_back(buf);
		}

		if (Sub(ss)) {
			cout << "YES" << endl;
			return 0;
		}
	}
	cout << "NO" << endl;
	return 0;
}


Measured memory_bytes: 1212