#include <bits/stdc++.h>
#define ADD(a, b) a = (a + ll(b)) % mod
#define MUL(a, b) a = (a * ll(b)) % mod
#define MAX(a, b) a = max(a, b)
#define MIN(a, b) a = min(a, b)
#define rep(i, a, b) for(int i = int(a); i < int(b); i++)
#define rer(i, a, b) for(int i = int(a) - 1; i >= int(b); i--)
#define all(a) (a).begin(), (a).end()
#define sz(v) (int)(v).size()
#define pb push_back
#define sec second
#define fst first
#define debug(fmt, ...) Debug(__LINE__, ":", fmt, ##__VA_ARGS__)
using namespace std;
typedef long long ll;
typedef unsigned long long ull;
typedef pair<int, int> pi;
typedef pair<ll, ll> pl;
typedef pair<int, pi> ppi;
typedef vector<int> vi;
typedef vector<ll> vl;
typedef vector<vl> mat;
typedef complex<double> comp;
void Debug() {cout << '\n'; }
template<class FIRST, class... REST>void Debug(FIRST arg, REST... rest){
	cout<<arg<<" ";Debug(rest...);}
template<class T>ostream& operator<<(ostream& out,const vector<T>& v) {
	out<<"[";if(!v.empty()){rep(i,0,sz(v)-1)out<<v[i]<<", ";out<<v.back();}out<<"]";return out;}
template<class S, class T>ostream& operator<<(ostream& out,const pair<S, T>& v){
	out<<"("<<v.first<<", "<<v.second<<")";return out;}
const int MAX_N = 200010;
const int MAX_V = 100010;
const double eps = 1e-6;
const ll mod = 1000000007;
const int inf = 1 << 29;
const ll linf = 1LL << 60;
const double PI = 3.14159265358979323846;
///////////////////////////////////////////////////////////////////////////////////////////////////
 
int H, W;
char S[12][12];
pi P[12];
bool visited[12];
bool same[12][12];

int creek(int v) {
	visited[v] = true;
	int res = 1;
	rep(i, 0, W) {
		if(!visited[i] && same[v][i]) {
			res += creek(i);
		}
	}
	return res;
}

bool ok() {
	// debug(vector<pi>(P, P + (H + 1) / 2));
	char T[12][12];
	int m1 = (H - 1) / 2, m2 = H / 2;
	rep(i, 0, (H + 1) / 2) {
		int a = P[i].fst, b = P[i].sec;
		rep(j, 0, W) {
			T[m1 - i][j] = S[a][j];
			T[m2 + i][j] = S[b][j];
		}
	}

	bool self[12];
	memset(same, 0, sizeof(same));
	memset(visited, 0, sizeof(visited));
	memset(self, 0, sizeof(self));

	rep(i, 0, W) {
		rep(j, 0, W) {
			bool ok = true;
			rep(k, 0, H) {
				if(T[H - k - 1][i] != T[k][j]) {
					ok = false;
					break;
				}
			}
			same[i][j] = ok;
		}
	}
	// rep(i, 0, W) {
	// 	rep(j, 0, W) {
	// 		cout << same[i][j];
	// 	}
	// 	cout << "\n";
	// }
	rep(i, 0, W) {
		if(same[i][i]) self[i] = true;
	}
	rep(i, 0, W) {
		if(self[i] || visited[i]) continue;
		if(creek(i) % 2) return false;
	}
	int odd = 0;
	rep(i, 0, W) {
		if(visited[i]) continue;
		odd += creek(i) % 2;
	}
	return odd <= 1;
}


bool used[12];

bool loop(int k) {
	if(k == (H + 1) / 2) {
		if(ok()) return true;
		else return false;
	}
	if(H % 2 == 1 && k == 0) {
		bool res = false;
		rep(i, 0, H) {
			P[k] = pi(i, i);
			used[i] = true;
			res |= loop(k + 1);
			used[i] = false;
		}
		return res;
	}
	bool res = false;
	int at = -1;
	rep(i, 0, H) {
		if(used[i]) continue;
		at = i;
		break;
	}
	used[at] = true;
	rep(i, 0, H) {
		if(used[i]) continue;
		used[i] = true;
		P[k] = pi(at, i);
		res |= loop(k + 1);
		used[i] = false;
	}
	used[at] = false;
	return res;
}

void solve() {
	cin >> H >> W;
	rep(i, 0, H) {
		rep(j, 0, W) {
			cin >> S[i][j];
		}
	}
	if(loop(0)) cout << "YES\n";
	else cout << "NO\n";
}
 
int main() {
#ifndef LOCAL
	ios::sync_with_stdio(false);
    cin.tie(0);
#endif
    cout << fixed;
	cout.precision(20);
	srand((unsigned int)time(NULL));
#ifdef LOCAL
	//freopen("in.txt", "wt", stdout); //for tester
    freopen("in.txt", "rt", stdin);
#endif	
	solve();
#ifdef LOCAL
    cerr << "Time elapsed: " << 1.0 * clock() / CLOCKS_PER_SEC << " s.\n";
#endif
	return 0;
}
 


Measured memory_bytes: 256