#include "bits/stdc++.h"

using namespace std;
template<typename T1, typename T2>inline void chkmin(T1 &x, T2 y) { if (x > y) x = y; }
template<typename T1, typename T2>inline void chkmax(T1 &x, T2 y) { if (x < y) x = y; }
template<typename T, typename U> inline ostream &operator<< (ostream &_out, const pair<T, U> &_p) { _out << _p.first << ' ' << _p.second; return _out; }
template<typename T, typename U> inline istream &operator>> (istream &_in, pair<T, U> &_p) { _in >> _p.first >> _p.second; return _in; }
template<typename T> inline ostream &operator<< (ostream &_out, const vector<T> &_v) { if (_v.empty()) { return _out; } _out << _v.front(); for (auto _it = ++_v.begin(); _it != _v.end(); ++_it) { _out << ' ' << *_it; } return _out; }
template<typename T> inline istream &operator>> (istream &_in, vector<T> &_v) { for (auto &_i : _v) { _in >> _i; } return _in; }
template<typename T> inline ostream &operator<< (ostream &_out, const set<T> &_s) { if (_s.empty()) { return _out; } _out << *_s.begin(); for (auto _it = ++_s.begin(); _it != _s.end(); ++_it) { _out << ' ' << *_it; } return _out; }
template<typename T> inline ostream &operator<< (ostream &_out, const multiset<T> &_s) { if (_s.empty()) { return _out; } _out << *_s.begin(); for (auto _it = ++_s.begin(); _it != _s.end(); ++_it) { _out << ' ' << *_it; } return _out; }
template<typename T> inline ostream &operator<< (ostream &_out, const unordered_set<T> &_s) { if (_s.empty()) { return _out; } _out << *_s.begin(); for (auto _it = ++_s.begin(); _it != _s.end(); ++_it) { _out << ' ' << *_it; } return _out; }
template<typename T> inline ostream &operator<< (ostream &_out, const unordered_multiset<T> &_s) { if (_s.empty()) { return _out; } _out << *_s.begin(); for (auto _it = ++_s.begin(); _it != _s.end(); ++_it) { _out << ' ' << *_it; } return _out; }
template<typename T, typename U> inline ostream &operator<< (ostream &_out, const map<T, U> &_m) { if (_m.empty()) { return _out; } _out << '(' << _m.begin()->first << ": " << _m.begin()->second << ')'; for (auto _it = ++_m.begin(); _it != _m.end(); ++_it) { _out << ", (" << _it->first << ": " << _it->second << ')'; } return _out; }
template<typename T, typename U> inline ostream &operator<< (ostream &_out, const unordered_map<T, U> &_m) { if (_m.empty()) { return _out; } _out << '(' << _m.begin()->first << ": " << _m.begin()->second << ')'; for (auto _it = ++_m.begin(); _it != _m.end(); ++_it) { _out << ", (" << _it->first << ": " << _it->second << ')'; } return _out; }
#define sz(c) (int)(c).size()
#define all(c) (c).begin(), (c).end()
#define rall(c) (c).rbegin(), (c).rend()
#define left left228
#define right right228
#define next next228
#define rank rank228
#define prev prev228
#define y1 y1228                                                         
#define read(FILENAME) freopen((FILENAME + ".in").c_str(), "r", stdin)
#define write(FILENAME) freopen((FILENAME + ".out").c_str(), "w", stdout)
#define files(FILENAME) read(FILENAME), write(FILENAME)
#define pb push_back
#define x first
#define y second   
const string FILENAME = "input";
//WULD! NAH! KEST!
const int MAXN = 20;
int h, w;
int p[MAXN];
string s[MAXN];
bool used[MAXN];
bool used1[MAXN];
bool was = false;
bool was1 = false;
int cnt = 0;
int heh = -2;
vector<int> gs;


void get1(int i) {
	if (i == w) {
		cout << "YES\n";
		exit(0);
	}
	if (used1[i]) {
		get1(i + 1);
	}
	else {
		for (int j = i + 1; j < w; j++) {
			if (!used1[j]) {
				used1[j] = true;
				used1[i] = true;
				bool bad = false;
				for (auto x : gs) {
					if (s[x][i] != s[p[x]][j]) {
						bad = true;
						break;
					}
					if (s[x][j] != s[p[x]][i]) {
						bad = true;
						break;
					}
				}
				if (heh != -2) {
					if (s[heh][i] != s[heh][j]) {
						bad = true;
					}
					if (s[heh][j] != s[heh][i]) {
						bad = true;
					}
				}
				if (!bad) {
					get1(i + 1);
				}
				used1[j] = false;
				used1[i] = false;
			}
		}
		if (w % 2 == 1 && !was1) {
			was1 = true;
			bool bad = false;
			for (auto x : gs) {
				if (s[x][i] != s[p[x]][i]) {
					bad = true;
					break;
				}
			}
			if (!bad) {
				get1(i + 1);
			}
			was1 = false;
		}
	}
}

void check() {
	get1(0);
}


void get(int i) {
	if (i == h) {
		check();
		return;
	}
	if (used[i]) {
		get(i + 1);
	}
	else {
		for (int j = i + 1; j < h; j++) {
			if (!used[j]) {
				used[j] = true;
				used[i] = true;
				p[i] = j;
				p[j] = i;
				gs.pb(i);
				get(i + 1);
				gs.pop_back();
				used[j] = false;
				used[i] = false;
			}
		}
		if (h % 2 == 1 && !was) {
			was = true;
			p[i] = -1;
			heh = i;
			get(i + 1);
			was = false;
		}
	}
}


int main() {
	ios::sync_with_stdio(0);
	cin.tie(0);
	cout.tie(0);
	// read(FILENAME);
	cin >> h >> w;
	for (int i = 0; i < h; i++) {
		cin >> s[i];
	}
	get(0);
	cout << "NO\n";
	return 0;
}

Measured memory_bytes: 256