// includes
#include <bits/stdc++.h>

// macros
#define ll long long int
#define pb emplace_back
#define mk make_pair
#define pq priority_queue
#define FOR(i, a, b) for(int i=(a);i<(b);++i)
#define rep(i, n) FOR(i, 0, n)
#define rrep(i, n) for(int i=((int)(n)-1);i>=0;i--)
#define irep(itr, st) for(auto itr = (st).begin(); itr != (st).end(); ++itr)
#define irrep(itr, st) for(auto itr = (st).rbegin(); itr != (st).rend(); ++itr)
#define vrep(v, i) for(int i = 0; i < (v).size(); i++)
#define all(x) (x).begin(),(x).end()
#define sz(x) ((int)(x).size())
#define UNIQUE(v) v.erase(unique(v.begin(), v.end()), v.end())
#define FI first
#define SE second
#define dump(a, n) for(int i = 0; i < n; i++)cout << a[i] << "\n "[i + 1 != n];
#define dump2(a, n, m) for(int i = 0; i < n; i++)for(int j = 0; j < m; j++)cout << a[i][j] << "\n "[j + 1 != m];
#define bit(n) (1LL<<(n))
#define INT(n) int n; cin >> n;
#define LL(n) ll n; cin >> n;
#define DOUBLE(n) double n; cin >> n;
using namespace std;

template <class T>bool chmax(T &a, const T &b){if(a < b){a = b; return 1;} return 0;}
template <class T>bool chmin(T &a, const T &b){if(a > b){a = b; return 1;} return 0;}
template <typename T> istream &operator>>(istream &is, vector<T> &vec){for(auto &v: vec)is >> v; return is;}
template <typename T> ostream &operator<<(ostream &os, const vector<T>& vec){for(int i = 0; i < vec.size(); i++){ os << vec[i]; if(i + 1 != vec.size())os << " ";} return os;}
template <typename T> ostream &operator<<(ostream &os, const set<T>& st){for(auto itr = st.begin(); itr != st.end(); ++itr){ os << *itr; auto titr = itr; if(++titr != st.end())os << " ";} return os;}
template <typename T> ostream &operator<<(ostream &os, const unordered_set<T>& st){for(auto itr = st.begin(); itr != st.end(); ++itr){ os << *itr; auto titr = itr; if(++titr != st.end())os << " ";} return os;}
template <typename T> ostream &operator<<(ostream &os, const multiset<T>& st){for(auto itr = st.begin(); itr != st.end(); ++itr){ os << *itr; auto titr = itr; if(++titr != st.end())os << " ";} return os;}
template <typename T> ostream &operator<<(ostream &os, const unordered_multiset<T>& st){for(auto itr = st.begin(); itr != st.end(); ++itr){ os << *itr; auto titr = itr; if(++titr != st.end())os << " ";} return os;}
template <typename T1, typename T2> ostream &operator<<(ostream &os, const pair<T1, T2> &p){os << p.first << " " << p.second; return os;}
template <typename T1, typename T2> ostream &operator<<(ostream &os, const map<T1, T2> &mp){for(auto itr = mp.begin(); itr != mp.end(); ++itr){ os << itr->first << ":" << itr->second; auto titr = itr; if(++titr != mp.end())os << " "; } return os;}
template <typename T1, typename T2> ostream &operator<<(ostream &os, const unordered_map<T1, T2> &mp){for(auto itr = mp.begin(); itr != mp.end(); ++itr){ os << itr->first << ":" << itr->second; auto titr = itr; if(++titr != mp.end())os << " "; } return os;}

//  types
typedef pair<int, int> P;
typedef pair<ll, int> Pl;
typedef pair<ll, ll> Pll;
typedef pair<double, double> Pd;
typedef complex<double> cd;

// constants
const int inf = 1e9;
const ll linf = 1LL << 50;
const double EPS = 1e-10;
const int mod = 1e9 + 7;
const int dx[4] = {-1, 0, 1, 0};
const int dy[4] = {0, -1, 0, 1};

// solve
int h, w;
string s[12];
bool vis[12];
vector<int> v;
string t[12], revt[12];

bool dfs(int i){
  if(i == h){
    rep(j, w){
      t[j] = "";
      for(int k = 0; k < h; k += 2){
        t[j] += s[v[k]][j];
      }
      string tmp = "";
      for(int k = 1; k < h; k += 2){
        tmp += s[v[k]][j];
      }
      reverse(all(tmp));
      t[j] += tmp;
      revt[j] = t[j]; reverse(all(revt[j]));
    }
    vector<bool> ok(w, false);
    int cnt = 0;
    rep(j, w){
      int fal = 0; rep(l, w)if(!ok[l])fal++;
      if(fal < 2)break;
      int k;
      for(k = 0; k < w; k++){
        if(!ok[k])break;
      }
      ok[k] = true;
      bool res = false;
      for(int l = k + 1; l < w; l++){
        if(!ok[l] && t[k] == revt[l]){
          res = true;
          ok[l] = true;
          break;
        }
      }
      if(!res){
        cnt++;
        if(cnt > (w % 2))return false;
      }
    }
    return true;
  }
  if(i % 2 == 0){
    bool res = false;
    int j = 0;
    while(vis[j])j++;
    vis[j] = true;
    v.pb(j);
    res |= dfs(i + 1);
    if(res)return true;
    v.pop_back();
    vis[j] = false;
    if(h % 2 == 1){
      j++;
      while(j < h && vis[j])j++;
      if(j != h){
        vis[j] = true;
        v.pb(j);
        res |= dfs(i + 1);
        if(res)return true;
        v.pop_back();
        vis[j] = false;
      }
    }
    return res;
  }else{
    int st = v[sz(v) - 1];
    FOR(j, st + 1, h){
      if(vis[j])continue;
      vis[j] = true;
      v.pb(j);
      bool res = dfs(i + 1);
      if(res)return true;
      v.pop_back();
      vis[j] = false;
    }
    return false;
  }
}

int main(int argc, char const* argv[])
{
  ios_base::sync_with_stdio(false);
  cin.tie(0);
  cout << fixed << setprecision(20);
  cin >> h >> w;
  rep(i, h)cin >> s[i];
  if(dfs(0))cout << "YES" << endl;
  else cout << "NO" << endl;
  return 0;
}


Measured memory_bytes: 256