// 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;

//  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
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;}

int h, w;
string s[14];
bool used[14];
vector<int> v;

bool dfs(int i){
  if(i == h / 2){
    vector<string> t(h, string(w, '0'));
    rep(j, sz(v)){
      if(j % 2 == 0){
        t[j/2] = s[v[j]];
      }else{
        t[h-1-j/2] = s[v[j]];
      }
    }
    if(h % 2 == 1){
      rep(j, h){
        if(!used[j]){
          t[h/2] = s[j];
          break;
        }
      }
    }
    vector<string> c(w, string(h, '0'));
    rep(j, w){
      rep(k, h){
        c[j][k] = t[k][j];
      }
    }
    vector<bool> ok(w, false);
    int cnt = 0;
    vector<string> rem;
    rep(j, w){
      if(!ok[j]){
        ok[j] = true;
        bool flag = false;
        rep(k, w){
          if(j != k && !ok[k]){
            string tmp = c[k];
            reverse(all(tmp));
            if(tmp == c[j]){
              flag = true;
              ok[k] = true;
              break;
            }
          }
        }
        if(!flag){
          cnt++;
          rem.pb(c[j]);
        }
      }
    }
    if(cnt == 0)return true;
    if(cnt == w % 2){
      string tmp = rem[0];
      reverse(all(tmp));
      if(tmp == rem[0])return true;
    }
    return false;
  }
  int cc = 0;
  rep(j, h){
    if(!used[j]){
      used[j] = true;
      v.pb(j);
      FOR(k, j + 1, h){
        if(!used[k]){
          used[k] = true;
          v.pb(k);
          if(dfs(i+1))return true;
          used[k] = false;
          v.pop_back();
        }
      }
      used[j] = false;
      v.pop_back();
      if(cc == 1)break;
      cc = 1;
    }
  }
  return false;
}

int main(int argc, char const* argv[])
{
  ios_base::sync_with_stdio(false);
  cin.tie(0);
  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