#include <iostream>
#include <queue>
#include <map>
#include <list>
#include <vector>
#include <string>
#include <stack>
#include <limits>
#include <climits>
#include <cassert>
#include <fstream>
#include <cstring>
#include <cmath>
#include <bitset>
#include <iomanip>
#include <algorithm>
#include <functional>
#include <cstdio>
#include <ciso646>
#include <set>
#include <array>
#include <unordered_map>

using namespace std;

#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<(n);i++)
#define RREP(i,n) for (int i=(n)-1;i>=0;i--)

#define inf 0x3f3f3f3f
#define PB push_back
#define MP make_pair
#define ALL(a) (a).begin(),(a).end()
#define SET(a,c) memset(a,c,sizeof a)
#define CLR(a) memset(a,0,sizeof a)
#define VS vector<string>
#define VI vector<ll>
#define DEBUG(x) cout<<#x<<": "<<x<<endl
#define MIN(a,b) (a>b?b:a)
#define MAX(a,b) (a>b?a:b)
#define pi 2*acos(0.0)
#define INFILE() freopen("in0.txt","r",stdin)
#define OUTFILE()freopen("out0.txt","w",stdout)
#define ll long long
#define ull unsigned long long
#define pii pair<ll,ll>
#define pcc pair<char,char>
#define pic pair<ll,char>
#define pci pair<char,ll>
#define eps 1e-14
#define FST first
#define SEC second
#define SETUP cin.tie(0), ios::sync_with_stdio(false), cout << setprecision(15)

namespace {
	struct input_returnner {
		ll N; input_returnner(ll N_ = 0) :N(N_) {}
		template<typename T> operator vector<T>() const { vector<T> res(N); for (auto &a : res) cin >> a; return std::move(res); }
		template<typename T> operator T() const { T res; cin >> res; return res; }
		template<typename T> T operator - (T right) { return T(input_returnner()) - right; }
		template<typename T> T operator + (T right) { return T(input_returnner()) + right; }
		template<typename T> T operator * (T right) { return T(input_returnner()) * right; }
		template<typename T> T operator / (T right) { return T(input_returnner()) / right; }
		template<typename T> T operator << (T right) { return T(input_returnner()) << right; }
		template<typename T> T operator >> (T right) { return T(input_returnner()) >> right; }
	};
	template<typename T> input_returnner in() { return in<T>(); }
	input_returnner in() { return input_returnner(); }
	input_returnner in(ll N) { return std::move(input_returnner(N)); }
}

const ll MOD = 1e9 + 7;

bool solve();

signed main() {
	SETUP;
	cout << (solve() ? "YES" : "NO") << endl;
#ifdef _DEBUG
	system("pause");
#endif
	return 0;
}

#define int ll

vector<string> solve2(int H, int W, vector<string> strs) {
	vector<string> org;
	vector<string> ans(H);
	vector<pair<string, int>> S;
	map<string, vector<int>> ms;
	REP(i, H) {
		S.push_back(MP(strs[i], i));
		org.push_back(strs[i]);
	}
	REP(i, H) {
		sort(ALL(S[i].first));
	}
	sort(ALL(S));
	REP(i, H) {
		ms[S[i].first].push_back(i);
	}

	bool f = ((H%2) != 0);
	int cnt = 0;
	for (auto &a : ms) {
		if (a.second.size() % 2 == 1) {
			if (f) {
				f = false;
				ans[H/2] = org[S[a.second[0]].second];
				for (int i = 1; i < a.second.size();) {
					ans[cnt] = org[a.second[i++]];
					ans[H-cnt - 1] = org[S[a.second[i++]].second];
					cnt++;
				}
			}
			else return{};
		}
		else {
			for (int i = 0; i < a.second.size();) {
				ans[cnt] = org[S[a.second[i]].second]; i++;
				ans[H - cnt - 1] = org[S[a.second[i]].second]; i++;
				cnt++;
			}
		}
	}
	return ans;
}

bool solve() {
	int H, W; cin >> H >> W;
	vector<string> strs;
	vector<string> tstrs;
	REP(i, H) {
		string s; cin >> s;
		strs.push_back(s);
	}
	strs = solve2(H, W, strs);
	if (strs.size() == 0) return false;
	REP(i, W) {
		string str;
		REP(j, H) {
			str += strs[j][i];
		}
		tstrs.push_back(str);
	}
	tstrs = solve2(W, H, tstrs);
	if (tstrs.size() == 0) return false;
	REP(i, W) {
		REP(j, H) {
			if (tstrs[i][j] != tstrs[W - i - 1][H - j - 1]) return false;
		}
	}
	return true;
}


Measured memory_bytes: 256