#include <bits/stdc++.h>

using namespace std;
typedef long long ll;
typedef pair<ll, ll> p_ll;

template<class T>
void debug(T itr1, T itr2) { auto now = itr1; while(now<itr2) { cout << *now << " "; now++; } cout << endl; }
#define repr(i,from,to) for (int i=(int)from; i<(int)to; i++)
#define all(vec) vec.begin(), vec.end()
#define rep(i,N) repr(i,0,N)
#define per(i,N) for (int i=(int)N-1; i>=0; i--)

const ll MOD = pow(10,9)+7;
const ll LLINF = pow(2,61)-1;
const int INF = pow(2,30)-1;

vector<ll> fac;
void c_fac(int x=pow(10,6)+10) { fac.resize(x,true); rep(i,x) fac[i] = i ? (fac[i-1]*i)%MOD : 1; }
ll inv(ll a, ll m=MOD) { ll b = m, x = 1, y = 0; while (b!=0) { int d = a/b; a -= b*d; swap(a,b); x -= y*d; swap(x,y); } return (x+m)%m; }
ll nck(ll n, ll k) { return fac[n]*inv(fac[k]*fac[n-k]%MOD)%MOD; }
ll gcd(ll a, ll b) { if (a<b) swap(a,b); return b==0 ? a : gcd(b, a%b); }
ll lcm(ll a, ll b) { return a/gcd(a,b)*b; }

int H, W;
vector<string> ts, S;
string result = "NO";
vector<bool> usedw;
vector<vector<bool>> okw;

bool checkh(int l, int r) {
  int ftl[26][26] = {}, ftr[26][26] = {};
  rep(i,W) {
    int nl = ts[l][i]-'a', nr = ts[r][i]-'a';
    ftl[nl][nr]++; ftr[nr][nl]++;
  }
  bool result = true;
  rep(i,26) rep(j,26) result &= ftl[i][j] == ftr[i][j];
  return result;
}

bool symh(int p) {
  int count[26] = {}; rep(i,W) count[ts[p][i]-'a']++;
  int x = 0; rep(i,26) x += count[i]%2;
  return x<=1;
}

// -------------------------------------------

bool checkw(int l, int r) {
  bool result = true;
  rep(i,H) result *= S[i][l]==S[(H-1)-i][r];
  return result;
}

bool dfs(int count=0) {
  int l = count; while (l<W&&usedw[l]) l++;
  int r = l+1; while (r<W&&usedw[r]) r++;
  if (l==W) return true;
  if (r==W) return checkw(l,r);
  bool result = false;
  usedw[l] = true;
  repr(i,r,W) {
    if (usedw[i]==true) continue;
    if (!okw[l][i]) continue;
    usedw[i] = true;
    // cout << count << "->" << l << " " << i << endl;
    result |= dfs(count+1);
    usedw[i] = false;
  }
  usedw[l] = false;
  return result;

}

int main() {
  cin >> H >> W; S.resize(H); usedw.resize(W,false);
  ts.resize(H); rep(i,H) cin >> ts[i];
  int count[H][26] = {}; rep(i,H) rep(j,W) count[i][ts[i][j]-'a']++;
  bool used[H] = {};
  int oddcount = H%2, posh = 0, counth = 0;
  rep(posh,H) {
    if (used[posh]) continue;
    used[posh] = true;
    int tposh = posh+1; while (tposh<H&&(used[tposh]||!checkh(posh,tposh))) tposh++;
    if (tposh==H) {
      if (oddcount==0||!symh(posh)) { cout << "NO" << endl; return 0; }
      else { S[H/2] = ts[posh]; oddcount = 0; }
    }
    else {
      S[counth] = ts[posh]; S[(H-1)-counth] = ts[tposh];
      used[tposh] = true; counth++;
    }
  }
  // rep(i,H) cout << S[i] << " " << S[H-1-i] << endl;
  okw.resize(W); rep(i,W) okw[i].resize(W,true);
  rep(i,W) rep(j,W) okw[i][j] = checkw(i,j);
  // rep(i,W) debug(all(okw[i]));
  if (W%2==0) {
    result = dfs() ? "YES" : "NO";
  }
  else {
    bool ok = false;
    rep(i,W) {
      if (okw[i][i]) {
        usedw[i] = true;
        ok |= dfs();
        usedw[i] = false;
      }
      result = ok ? "YES" : "NO";
    }
  }
  cout << result << endl;
  return 0;
}

Measured memory_bytes: 256