#include <bits/stdc++.h>
using namespace std;
using ll = long long;
using ull = unsigned long long;

#define REP(i,n) for(long long i = 0; i < (n); i++)
#define FOR(i, m, n) for(long long i = (m);i < (n); ++i)
#define ALL(obj) (obj).begin(),(obj).end()
#define SPEED cin.tie(0);ios::sync_with_stdio(false);

template<class T> using V = vector<T>;
template<class T, class U> using P = pair<T, U>;
template<class T> using PQ = priority_queue<T>;
template<class T> using PQR = priority_queue<T,vector<T>,greater<T>>;

constexpr long long MOD = (long long)1e9 + 7;
constexpr long long MOD2 = 998244353;
constexpr long long HIGHINF = (long long)1e18;
constexpr long long LOWINF = (long long)1e15;
constexpr long double PI = 3.1415926535897932384626433;

template <class T> vector<T> multivector(size_t N,T init){return vector<T>(N,init);}
template <class... T> auto multivector(size_t N,T... t){return vector<decltype(multivector(t...))>(N,multivector(t...));}
template <class T> void corner(bool flg, T hoge) {if (flg) {cout << hoge << endl; exit(0);}}
template <class T, class U>ostream &operator<<(ostream &o, const map<T, U>&obj) {o << "{"; for (auto &x : obj) o << " {" << x.first << " : " << x.second << "}" << ","; o << " }"; return o;}
template <class T>ostream &operator<<(ostream &o, const set<T>&obj) {o << "{"; for (auto itr = obj.begin(); itr != obj.end(); ++itr) o << (itr != obj.begin() ? ", " : "") << *itr; o << "}"; return o;}
template <class T>ostream &operator<<(ostream &o, const multiset<T>&obj) {o << "{"; for (auto itr = obj.begin(); itr != obj.end(); ++itr) o << (itr != obj.begin() ? ", " : "") << *itr; o << "}"; return o;}
template <class T>ostream &operator<<(ostream &o, const vector<T>&obj) {o << "{"; for (int i = 0; i < (int)obj.size(); ++i)o << (i > 0 ? ", " : "") << obj[i]; o << "}"; return o;}
template <class T, class U>ostream &operator<<(ostream &o, const pair<T, U>&obj) {o << "{" << obj.first << ", " << obj.second << "}"; return o;}
template <template <class tmp>  class T, class U> ostream &operator<<(ostream &o, const T<U> &obj) {o << "{"; for (auto itr = obj.begin(); itr != obj.end(); ++itr)o << (itr != obj.begin() ? ", " : "") << *itr; o << "}"; return o;}
void print(void) {cout << endl;}
template <class Head> void print(Head&& head) {cout << head;print();}
template <class Head, class... Tail> void print(Head&& head, Tail&&... tail) {cout << head << " ";print(forward<Tail>(tail)...);}
template <class T> void chmax(T& a, const T b){a=max(a,b);}
template <class T> void chmin(T& a, const T b){a=min(a,b);}
void YN(bool flg) {cout << (flg ? "YES" : "NO") << endl;}
void Yn(bool flg) {cout << (flg ? "Yes" : "No") << endl;}
void yn(bool flg) {cout << (flg ? "yes" : "no") << endl;}

vector<vector<pair<int,int>>> pa;
void dfs(set<int> st,vector<pair<int,int>> vp) {
	if(st.empty()) {
		pa.push_back(vp);
		return;
	}
	int l = *st.begin();
	st.erase(st.begin());
	for(auto e:st) {
		int r = e;
		auto tmpst = st;
		tmpst.erase(r);
		auto tmpvp = vp;
		tmpvp.push_back({l,r});
		dfs(tmpst,tmpvp);
	}
}

int main() {
	int H,W; cin >> H >> W;
	vector<string> S(H);
	for(int i = 0; i < H; ++i) cin >> S[i];
	set<int> tmpst;
	for(int i = 0; i < H; ++i) tmpst.insert(i);
	if(H%2) tmpst.insert(-1);
	vector<pair<int,int>> tmpvp;
	dfs(tmpst,tmpvp);
	for(auto& vp:pa) {
		vector<string> T(H);
		int idx = 0;
		for(auto p:vp) {
			if(p.first == -1 || p.second == -1) {
				T[H/2] = S[max(p.first,p.second)];
			}
			else {
				T[idx] = S[p.first];
				T[H-1-idx] = S[p.second];
				idx++;
			}
		}
		vector<string> U(W),X(W);
		for(int j = 0; j < W; ++j) for(int i = 0; i < H; ++i) U[j].push_back(T[i][j]);
		for(int j = 0; j < W; ++j) {
			X[j] = U[j];
			reverse(ALL(X[j]));
		}
		vector<int> c(W,0);
		int cnt = 0;
		int flg1 = 1;
		for(int j = 0; j < W && flg1; ++j) {
			if(c[j]) continue;
			for(int k = 0; k < W && !c[j]; ++k) {
				if(c[k] || j == k) continue;
				if(U[j] == X[k]) c[j] = c[k] = 1;
			}
			if(!c[j]) {
				if(U[j]==X[j]) {
					c[j] = 1;
					cnt++;
				}
				else {
					flg1 = 0;
					break;
				}
			}
		}
		if(W%2 && cnt!=1) continue;
		if(!(W%2) && cnt!=0) continue;
		corner(flg1,"YES");
	}
	cout << "NO" << endl;
	return 0;
}


Measured memory_bytes: 1152