//marico el que lo lea
#include <iostream>
#include <cstdio>
#include <cstring>
#include <cmath>
#include <algorithm>
#include <vector>
#include <map>
#include <set>
#include <queue>
#include <stack>
#include <stdlib.h>
#include <assert.h>
using namespace std;

typedef long long ll;
typedef vector<int> vi;
typedef vector<ll> vll;
typedef pair<int, int> ii;

void fastIO() {std::ios_base::sync_with_stdio(0);cin.tie(0);cout.tie(0);}

#define pb push_back
#define all(obj) obj.begin(), obj.end()
#define ms(obj, val) memset(obj, val, sizeof(obj))
#define ms2(obj, val, sz) memset(obj, val, sizeof(obj[0])*sz)

#define fst first
#define snd second

template<typename T, typename U> inline void mnze(T &x, U y) {if(y < x) x = y;}
template<typename T, typename U> inline void mxze(T &x, U y) {if(x < y) x = y;}

void _scan( int &x ) { scanf("%d",&x); }
void _scan( long long &x ) { scanf("%lld",&x); }
void _scan( double &x ) { scanf("%lf",&x); }
void _scan( char &x ) { scanf(" %c",&x); }
void _scan( char *x ) { scanf("%s",x); }
template<typename T>
void _scan( vector<T> &v ){
	for(int i=0; i<(int)v.size(); i++) _scan(v[i]);
}
void scan() {}
template<typename T, typename... U>
void scan( T& head, U&... tail ) { _scan(head); scan(tail...);}

template<typename T> void _dbg(const char* sdbg, T h)
	{ cerr<<sdbg<<"="<<h<<"\n"; }
template<typename T, typename... U> void _dbg(const char* sdbg, T h, U... t) {
	while(*sdbg != ',')cerr<<*sdbg++;
	cerr<<"="<<h<<","; _dbg(sdbg+1, t...);
}

#ifdef LOCAL
#define debug(...) _dbg(#__VA_ARGS__, __VA_ARGS__)
#define debugv(x) {{cerr<<#x<<" = "; for(auto _i:x) cerr<<_i<<", ";cerr<<"\n";}}
#define debuga(x, sz) {{cerr<<#x<<" = ";for(int _i=0;_i<sz;_i++)cerr<<x[_i]<<", ";cerr<<"\n";}}
#else
#define debug(...) (__VA_ARGS__)
#define debugv(x)
#define debuga(x, sz)
#define cerr if(0)cout
#endif

ll MOD = 1e9+7;

inline ll Msum(ll x){return x;}
template<typename... Rest>
inline ll Msum(ll x, Rest... rest){ return (x + Msum(rest...)) % MOD; }

inline ll Mprod(ll x){return x;}
template<typename... Rest>
inline ll Mprod(ll x, Rest... rest){ return x * Mprod(rest...) % MOD; }

inline ll Mnorm(ll x){ return (x%MOD + MOD) %MOD; }

inline ll Msq(ll x){ return (x*x) %MOD; }
inline ll Mpot(ll x, ll e){
	if(!e) return 1;
	return Msq(Mpot(x, e>>1)) * (e & 1 ? x : 1)%MOD;
}
////////////////////////////////////////////////////////////////////////////////
////////////////////////////////////////////////////////////////////////////////

const int MAXN = 13;

int R, C;
char S[MAXN][MAXN];

bool palim(string s){
	string s2=s;
	reverse(all(s));
	return s==s2;
}
bool checkCols(){
	//for(int r=0; r<R; r++){
		//printf("%s\n", S[r]);
	//}
	//printf("\n");
	if(C%2){
		string s;
		for(int r=0; r<R; r++) s = s+S[r][C/2];
		if(!palim(s)) return false;
	}
	map<string, int> m;
	for(int c=0; c<C; c++){
		if(C%2 && c==(C/2)) continue;

		string s;
		for(int r=0; r<R; r++) s = s+S[r][c];
		m[s]++;
	}
	for(auto it : m){
		if(palim(it.fst)){
			if(it.snd % 2) return false;
		}else{
			string aux = it.fst;
			reverse(all(aux));
			if(it.snd != m[aux]) return false;
		}
	}
	return true;
}

bool bt(int r){
	//debug(r, R);
	if(r >= R/2){
		return checkCols();
	}
	for(int ri=r+1; ri < R-r; ri++){
		//if(R%2 && ri == (R/2)) continue;
		swap(S[R-r-1], S[ri]);
		if(bt(r+1)) return true;

		swap(S[R-r-1], S[ri]);
	}
	return false;
}

int main(){
	scan(R, C);
	for(int r=0; r<R; r++) scan(S[r]);

	bool ans = false;
	if(C%2){
		for(int c=0; c<C && !ans; c++){
			for(int r=0; r<R; r++) swap(S[r][c], S[r][C/2]);
			if(R%2){
				for(int r=0; r<R && !ans; r++){
					swap(S[r], S[R/2]);
					ans |= bt(0);
					swap(S[r], S[R/2]);
				}
			}else{
				ans |= bt(0);
			}
			for(int r=0; r<R; r++) swap(S[r][c], S[r][C/2]);
		}
	}else{
		if(R%2){
			for(int r=0; r<R && !ans; r++){
				swap(S[r], S[R/2]);
				ans |= bt(0);
				swap(S[r], S[R/2]);
			}
		}else{
			ans |= bt(0);
		}
	}

	printf(ans ? "YES\n" : "NO\n");
}


Measured memory_bytes: 256