// Slower C++ solution:
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*/

/*****************

 *   I<3   C++   *

 *  IWANT ANYAC  *

 * ILOVEPROGRAM! *

 *IT'SINTERESTING*

 * ILOVEPROGRAM! *

 *  INCONTESTS   *

 *   GETSCORE    *

 *    ACCODE     *

 *     LETS      *

 *      GO       *

 *Written by:    *

 *    charlieyan *

 *****************/

#include <assert.h>

#include <ctype.h>

#include <errno.h>

#include <float.h>

#include <fstream>

#include <iomanip>

#include <iostream>

#include <limits>

#include <locale>

#include <math.h>

#include <stdio.h>

#include <stdlib.h>

#include <string.h>

#include <time.h>

#include <wchar.h>

#include <wctype.h>

#include <algorithm>

#include <bitset>

#include <cctype>

#include <cerrno>

#include <clocale>

#include <cmath>

#include <complex>

#include <cstdio>

#include <cstdlib>

#include <cstring>

#include <ctime>

#include <deque>

#include <exception>

#include <fstream>

#include <functional>

#include <limits>

#include <map>

#include <iomanip>

#include <ios>

#include <iosfwd>

#include <istream>

#include <ostream>

#include <queue>

#include <set>

#include <sstream>

#include <stack>

#include <stdexcept>

#include <streambuf>

#include <string>

#include <utility>

#include <vector>

#include <cwchar>

#include <cwctype>

#include <inttypes.h>

#include <stdbool.h>

#include <stdint.h>

#include <tgmath.h>

#include <numeric>

#include <list>

#define LL long long

#define pb push_back

#define pob pop_back

#define mp make_pair

#define FILL(a,v) memset((void*)a,v,sizeof(a))

#define FILL0(a) FILL(a,0)

#define FILL1(a) FILL(a,-1)

#define FILLBIG(a) FILL(a,0x7f)

#define ALL(a) (a).begin(),(a).end()

#define Inf ((int)0x7f7f7f7f)

#define INF ((long long)0x7f7f7f7f7f7f7f7f)

#define Mod 1000000007

#define foreach(it,x) for(__typeof(x.begin()) it=x.begin();it!=x.end();it++)

#define FOR(i,x) for(int i=0;i<x;i++)

#define RFOR(i,x) for(int i=x-1;i>=0;i--)

#define FFOR(i,a,b) for(int i=a;i<b;i++)

#define FRFOR(i,a,b) for(int i=a-1;i>=b;i--)

#define SZ(a) ((int)(a).size())

#define FI first

#define SE second

#define OVER(a) return puts(a),0

#define fastio ios::sync_with_stdio(0)

#define filei(f) freopen(f,"r",stdin)

#define fileo(f) freopen(f,"w",stdout)

#define eprintf(...) fprintf(stderr,__VA_ARGS__)

#define MASK(a) (1<<(a))

#define MASKX(x,a) ((x)<<(a))

#define BIT(a,b) ((a>>b)&1)

#define cddx(a) priority_queue<a >

#define cxdd(a) priority_queue<a,vector<a >,greater<a > >

#define deval(a) {cerr<<#a<<" = "<<a<<endl;}

#define dearr(be,en) {cerr<<#be;cerr<<" = {";for(auto i=be;i!=en;i++)cerr<<' '<<*i;cerr<<" }\n";}

#define reg register

#define prtime eprintf("\n\nTime used: %dms\n",clock());

#define PI 3.1415926535897932384626

#define TODEG(a) (a*180/PI)

//Lissin Chang loves Yang Shumeng

using namespace std;

typedef pair<int,int> pii;

typedef unsigned long long UINT;

typedef unsigned int uint;

typedef long long INT;

typedef long double ldouble;

typedef pair<INT,INT> PII;

typedef stringstream sst;

typedef istringstream iss;

typedef vector<int> vint;

typedef vector<INT> VINT;

typedef vector<pii> vpii;

typedef vector<PII> VPII;

typedef priority_queue<int> pqi;

typedef priority_queue<INT> PQI;

typedef priority_queue<pii> pqp;

typedef priority_queue<PII> PQP;

typedef priority_queue<int,vint,greater<int> > xqi;

typedef priority_queue<INT,VINT,greater<INT> > XQI;

typedef priority_queue<pii,vpii,greater<pii> > xqp;

typedef priority_queue<PII,VPII,greater<PII> > XQP;

//Coded by Charlieyan

//版权所有，翻印必究



int n, m;

char s[17][17];

string t[17];

int h[17];

int l[17];



void dfs1(int now,int ji)

{

	if(now==m)

	{

		puts("YES");

		exit(0);

	}

	if(l[now]!=-1)

	{

		dfs1(now + 1, ji);

		return;

	}

	for (int i = now + 1; i < m; i++)

	{

		if(l[i]!=-1)

			continue;

		bool can = 1;

		for (int j = 0; j < n; j++)

			if(s[j][now]!=s[h[j]][i])

				can = 0;

		if(!can)

			continue;

		l[now] = i;

		l[i] = now;

		dfs1(now + 1, ji);

		l[now] = l[i] = -1;

	}

	if(ji)

	{

		bool can = 1;

		for (int j = 0; j < n; j++)

			if(s[j][now]!=s[h[j]][now])

				can = 0;

		if(can)

		{

			l[now] = now;

			dfs1(now + 1, 0);

			l[now] = -1;

		}

	}

}



void dfs2(int now,int ji)

{

	if(now==n)

	{

		dfs1(0, m % 2);

		return;

	}

	if(h[now]!=-1)

	{

		dfs2(now + 1, ji);

		return;

	}

	for (int i = now + 1; i < n; i++)

	{

		if(h[i]!=-1)

			continue;

		h[now] = i;

		h[i] = now;

		dfs2(now + 1, ji);

		h[now] = h[i] = -1;

	}

	if(ji)

	{

		h[now] = now;

		dfs2(now + 1, 0);

		h[now] = -1;

	}

}



int main()

{

	cin >> n >> m;

	for (int i = 0; i < n; i++)

	{

		cin >> s[i];

		t[i] = s[i];

		sort(t[i].begin(), t[i].end());

	}

	FILL1(h);

	FILL1(l);

	dfs2(0, n % 2);

	OVER("NO");

}

// Faster C++ solution for the same problem (measured speedup: 3.011774788137035x):
#include <bits/stdc++.h>

using namespace std;

/*

Coded by 秦惜梦

The most attractive girl in the world

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*/



int n,m;

char s[15][15];

int cnt[15][35];

int tr[15],tc[15];



void colstrs(int now,bool js=m&1){

	if(now==m){

		puts("YES");

		exit(0);

	}

	if(~tc[now]){

		colstrs(now+1,js);

		return;

	}

	if(js){

		bool can=1;

		for(int j=0;j<n;j++){

			can&=(s[j][now]==s[tr[j]][now]);

		}

		if(can){

			tc[now]=now;

			colstrs(now+1,0);

			tc[now]=-1;

		}

	}

	for(int i=0;i<m;i++){

		if(i!=now&&!~tc[i]){

			bool can=1;

			for(int j=0;j<n;j++){

				can&=(s[j][now]==s[tr[j]][i]);

			}

			if(!can){

				continue;

			}

			tc[now]=i;

			tc[i]=now;

			colstrs(now+1,js);

			tc[now]=tc[i]=-1;

		}

	}

}



void rowstrs(int now,bool js=n&1){

	if(now==n){

		memset(tc,-1,sizeof(tc));

		colstrs(0);

		return;

	}

	if(~tr[now]){

		rowstrs(now+1,js);

		return;

	}

	if(js){

		tr[now]=now;

		rowstrs(now+1,0);

		tr[now]=-1;

	}

	for(int i=0;i<n;i++){

		if(i!=now&&!~tr[i]){

			bool can=1;

			for(int j=0;j<26;j++){

				can&=(cnt[now][j]==cnt[i][j]);

			}

			if(!can){

				continue;

			}

			tr[now]=i;

			tr[i]=now;

			rowstrs(now+1,js);

			tr[now]=tr[i]=-1;

		}

	}

}



int main(){

	scanf("%d%d",&n,&m);

	for(int i=0;i<n;i++){

		scanf("%s",s[i]);

	}

	for(int i=0;i<n;i++){

		for(int j=0;j<m;j++){

			cnt[i][s[i][j]-'a']++;

		}

	}

	memset(tr,-1,sizeof(tr));

	rowstrs(0);

	puts("NO");

	return 0;

}