// need
#include <iostream>
#include <algorithm>

// data structure
#include <bitset>
#include <map>
#include <queue>
#include <set>
#include <stack>
#include <string>
#include <utility>
#include <vector>
#include <complex>
//#include <deque>
#include <valarray>

// stream
//#include <istream>
//#include <sstream>
//#include <ostream>
#include <fstream>

// etc
#include <cassert>
#include <cmath>
#include <functional>
#include <iomanip>
#include <chrono>
#include <random>
#include <numeric>

// input
#define INIT std::ios::sync_with_stdio(false);std::cin.tie(0);
#define VAR(type, ...)type __VA_ARGS__;MACRO_VAR_Scan(__VA_ARGS__);
template<typename T> void MACRO_VAR_Scan(T& t) { std::cin >> t; }
template<typename First, typename...Rest>void MACRO_VAR_Scan(First& first, Rest&...rest) { std::cin >> first; MACRO_VAR_Scan(rest...); }
#define VEC_ROW(type, n, ...)std::vector<type> __VA_ARGS__;MACRO_VEC_ROW_Init(n, __VA_ARGS__); for(int i=0; i<n; ++i){MACRO_VEC_ROW_Scan(i, __VA_ARGS__);}
template<typename T> void MACRO_VEC_ROW_Init(int n, T& t) { t.resize(n); }
template<typename First, typename...Rest>void MACRO_VEC_ROW_Init(int n, First& first, Rest&...rest) { first.resize(n); MACRO_VEC_ROW_Init(n, rest...); }
template<typename T> void MACRO_VEC_ROW_Scan(int p, T& t) { std::cin >> t[p]; }
template<typename First, typename...Rest>void MACRO_VEC_ROW_Scan(int p, First& first, Rest&...rest) { std::cin >> first[p]; MACRO_VEC_ROW_Scan(p, rest...); }
#define VEC(type, c, n) std::vector<type> c(n);for(auto& i:c)std::cin>>i;
#define MAT(type, c, m, n) std::vector<std::vector<type>> c(m, std::vector<type>(n));for(auto& r:c)for(auto& i:r)std::cin>>i;

// output
#define OUT(d) std::cout<<(d);
#define FOUT(n, d) std::cout<<std::fixed<<std::setprecision(n)<<(d);
#define SOUT(n, c, d) std::cout<<std::setw(n)<<std::setfill(c)<<(d);
#define SP std::cout<<" ";
#define TAB std::cout<<"\t";
#define BR std::cout<<"\n";
#define SPBR(i, n) std::cout<<(i + 1 == n ? '\n' : ' ');
#define ENDL std::cout<<std::endl;
#define FLUSH std::cout<<std::flush;
#define SHOW(d) {std::cerr << #d << "\t:" << (d) << "\n";}
#define SHOWVECTOR(v) {std::cerr << #v << "\t:";for(const auto& xxx : v){std::cerr << xxx << " ";}std::cerr << "\n";}
#define SHOWVECTOR2(v) {std::cerr << #v << "\t:\n";for(const auto& xxx : v){for(const auto& yyy : xxx){std::cerr << yyy << " ";}std::cerr << "\n";}}
#define SHOWQUEUE(a) {auto tmp(a);std::cerr << #a << "\t:";while(!tmp.empty()){std::cerr << tmp.front() << " ";tmp.pop();}std::cerr << "\n";}

// utility
#define ALL(a) (a).begin(),(a).end()
#define FOR(i, a, b) for(int i=(a);i<(b);++i)
#define RFOR(i, a, b) for(int i=(b)-1;i>=(a);--i)
#define REP(i, n) for(int i=0;i<int(n);++i)
#define RREP(i, n) for(int i=int(n)-1;i>=0;--i)
#define FORLL(i, a, b) for(ll i=ll(a);i<ll(b);++i)
#define RFORLL(i, a, b) for(ll i=ll(b)-1;i>=ll(a);--i)
#define REPLL(i, n) for(ll i=0;i<ll(n);++i)
#define RREPLL(i, n) for(ll i=ll(n)-1;i>=0;--i)
#define IN(a, x, b) (a<=x && x<b)
template<typename T> inline T CHMAX(T& a, const T b) { return a = (a < b) ? b : a; }
template<typename T> inline T CHMIN(T& a, const T b) { return a = (a > b) ? b : a; }
#define EXCEPTION(msg) throw std::string("Exception : " msg " [ in ") + __func__ + " : " + std::to_string(__LINE__) + " lines ]"
#define TRY(cond, msg) try {if (cond) EXCEPTION(msg);}catch (std::string s) {std::cerr << s << std::endl;}
void CHECKTIME(std::function<void()> f) { auto start = std::chrono::system_clock::now(); f(); auto end = std::chrono::system_clock::now(); auto res = std::chrono::duration_cast<std::chrono::nanoseconds>((end - start)).count(); std::cerr << "[Time:" << res << "ns  (" << res / (1.0e9) << "s)]\n"; }

// test
template<class T> std::vector<std::vector<T>> VV(int n, int m, T init = T()) {
	return std::vector<std::vector<T>>(n, std::vector<T>(m, init));
}
template<typename S, typename T>
std::ostream& operator<<(std::ostream& os, std::pair<S, T> p) {
	os << "(" << p.first << ", " << p.second << ")"; return os;
}

// type/const
//#define int ll
using ll = long long;
using ull = unsigned long long;
using ld = long double;
using PAIR = std::pair<int, int>;
using PAIRLL = std::pair<ll, ll>;
constexpr int INFINT = 1 << 30;                          // 1.07x10^ 9
constexpr int INFINT_LIM = (1LL << 31) - 1;              // 2.15x10^ 9
constexpr ll INFLL = 1LL << 60;                          // 1.15x10^18
constexpr ll INFLL_LIM = (1LL << 62) - 1 + (1LL << 62);  // 9.22x10^18
constexpr double EPS = 1e-9;
constexpr int MOD = 1000000007;
constexpr double PI = 3.141592653589793238462643383279;

template<class T, size_t N> void FILL(T(&a)[N], const T& val) { for (auto& x : a) x = val; }
template<class ARY, size_t N, size_t M, class T> void FILL(ARY(&a)[N][M], const T& val) { for (auto& b : a) FILL(b, val); }
template<class T> void FILL(std::vector<T>& a, const T& val) { for (auto& x : a) x = val; }
template<class ARY, class T> void FILL(std::vector<std::vector<ARY>>& a, const T& val) { for (auto& b : a) FILL(b, val); }

// ------------>8------------------------------------->8------------

bool isPalindromic(const std::string& s) {
	size_t sn = s.size();
	size_t n = sn / 2;
	for (size_t i = 0; i < n; ++i) {
		if (s[i] != s[sn - i - 1]) return false;
	}
	return true;
}

bool check(int h, int w, std::vector<std::string> s) { // O(H!W!HW)
	return [&] {
		std::vector<int> v(h);
		std::iota(ALL(v), 0);
		do {
			std::vector<int> u(w);
			std::iota(ALL(u), 0);
			do {
				bool conti = [&] {
					REP(i, h) REP(j, w) {
						if (s[v[i]][u[j]] != s[v[h - i - 1]][u[w - j - 1]]) return true;
					}
					return false;
				}();
				if (conti) continue;
				else return true;
			} while (std::next_permutation(ALL(u)));
		} while (std::next_permutation(ALL(v)));
		return false;
	}();
}

bool solve(int h, int w, std::vector<std::string> s) {
	std::vector<int> vc(w); // ペア番号を格納した配列
	std::iota(ALL(vc), 0); // vc = {0,1,2,3,…,w-1}
	REP(j, w) vc[j] /= 2; // vc = {0,0,1,1,…,[(w-1)/2]}
	do { // vcを回していく
		{
			bool ok = true;
			int p = 0; // 今まで出てきた最大のペア番号
			REP(i, w) {
				if (w % 2 == 1 && vc[i] == w / 2) continue; // wが奇数のとき,ペアにならない列は除外
				if (vc[i] < p) continue; // p未満がすでに出てきているのは問題なし
				if (vc[i] == p) { ++p; continue; }
				ok = false; // vc[i]がpより大きいものは除外
				break;
			}
			if (!ok) continue;
		}
		std::vector<int> v(vc); // vcを用いて元の列vを構成する
		{
			std::vector<int> cnt((w + 1) / 2, 0); // ペア番号の出てきた数を格納する
			REP(j, w) {
				if (cnt[v[j]] == 0) ++cnt[v[j]]; // 初めに出てきたものはそのまま(列番号i)
				else v[j] = w - v[j] - 1; // 二回目に出てきたものは弄る(列番号w-i-1)
			}
		}

		std::map<std::string, int> map;
		REP(i, h) {
			std::string ss(w, ' ');
			REP(j, w) ss[v[j]] = s[i][j];
			++map[ss];
		}
		int odd = h % 2;
		bool ok = true;
		for (auto& x : map) {
			if (isPalindromic(x.first)) {
				if (x.second % 2 == 1) --odd;
				continue;
			}
			auto ss(x.first);
			std::reverse(ALL(ss));
			if (x.second != map[ss]) {
				ok = false;
				break;
			}
		}
		if (!ok || odd != 0) continue;
		return true;
	} while (std::next_permutation(ALL(vc)));
	return false;
}

signed main() {
	INIT;

	VAR(int, h, w);
	VEC(std::string, s, h);
	OUT(solve(h, w, s) ? "YES" : "NO")BR;
	return 0;
}

Measured memory_bytes: 256