//#pragma GCC target("avx2,avx")
#pragma GCC optimize("O3")
#pragma GCC optimize("unroll-loops")
//#include <atcoder/all>
//using namespace atcoder;
//using mint = modint998244353;
//using mint = modint1000000007;
#include <bits/stdc++.h>
using namespace std;
//#include <ext/pb_ds/assoc_container.hpp>
//#include <ext/pb_ds/tree_policy.hpp>
//using namespace __gnu_pbds;
//using i128 = __int128_t;
using ll = long long;
using ull = unsigned long long;
using pii = pair<int, int>;
using pll = pair<long long, long long>;
#define rep(i, n) for(int i = 0; i < (n); ++i)
#define all(x) (x).begin(),(x).end()
#define SZ(x) ((int)(x).size())
constexpr char ln = '\n';
template<class T1, class T2> inline bool chmax(T1 &a, T2 b) {if (a < b) {a = b; return true;} return false;}
template<class T1, class T2> inline bool chmin(T1 &a, T2 b) {if (a > b) {a = b; return true;} return false;}
inline int topbit(int x) {return x == 0 ? -1 : 31-__builtin_clz(x);}
inline int topbit(long long x) {return x == 0 ? -1 : 63-__builtin_clzll(x);}
inline int botbit(int x) {return x == 0 ? 32 : __builtin_ctz(x);}
inline int botbit(long long x) {return x == 0 ? 64 : __builtin_ctzll(x);}
inline int popcount(int x) {return __builtin_popcount(x);}
inline int popcount(long long x) {return __builtin_popcountll(x);}
inline int kthbit(long long x, int k) {return (x>>k)&1;}
inline void print() {cout << "\n";}
template<class T>
inline void print(const vector<T> &v) {
    for (auto itr = v.begin(); itr != v.end(); ++itr) cout << *itr << " ";
    print();
}
template<class T, class... Args>
inline void print(const T &x, const Args &... args) {
    cout << x << " ";
    print(args...);
}
#ifdef MINATO_LOCAL
#define dump(x) cerr << __LINE__ << " : " << #x << " = " << (x) << endl;
inline void debug() {cerr << endl;}
template<class T>
inline void debug(const vector<T> &v) {
    for (auto itr = v.begin(); itr != v.end(); ++itr) cerr << *itr << " ";
    debug();
}
template<class T, class... Args>
inline void debug(const T &x, const Args &... args) {
    cerr << x << " ";
    debug(args...);
}
#else
#define dump(x) void(0)
inline void debug() {}
template<class T> inline void debug(const vector<T> &v) {}
template<class T, class... Args> inline void debug(const T &x, const Args &... args) {}
#endif
struct Fast_ios {Fast_ios() {cin.tie(nullptr); ios::sync_with_stdio(false); cout << fixed << setprecision(20);};} fast_ios;
////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////

int main() {
    int H,W; cin >> H >> W;
    vector<string> S(H);
    rep(i,H) cin >> S[i];

    auto T = S;
    rep(i,H) sort(all(T[i]));
    vector<vector<int>> cnt(H, vector<int>(26));
    rep(i,H) rep(j,W) cnt[i][S[i][j]-'a']++;
    vector<string> U(W, string(H,' '));
    auto dfs=[&](auto&& self, int h)->void {
        if (h==H) {
            rep(i,H) {
                if (~i&1) {
                    rep(j,W) U[j][i/2] = S[H-1-i][j];
                } else {
                    rep(j,W) U[j][H-1-i/2] = S[H-1-i][j];
                }
            }
            bool center = W%2==1 ? false : true;
            for (int i = 0; i < W;) {
                auto str = U[i];
                reverse(all(str));
                bool ok = false;
                for (int j = i+1; j < W; j++) {
                    if (str==U[j]) {
                        ok = true;
                        swap(U[i+1],U[j]);
                        i += 2;
                        break;
                    }
                }
                if (!ok) {
                    if (!center and str==U[i]) {
                        i++;
                        center = true;
                    } else {
                        return;
                    }
                }
            }

            cout << "YES" << ln;
            exit(0);
        }
        for (int i = h+1; i < H; i++) {
            if (T[h]==T[i]) {
                swap(S[h+1],S[i]);
                swap(T[h+1],T[i]);
                self(self,h+2);
                swap(S[h+1],S[i]);
                swap(T[h+1],T[i]);
            }
        }

        return;
    };

    if (H&1) {
        rep(i,H) {
            int odd = 0;
            rep(j,26) {
                if (cnt[i][j]&1) odd++;
            }
            if (odd <= 1) {
                swap(S[0],S[i]);
                swap(T[0],T[i]);
                dfs(dfs,1);
                swap(S[0],S[i]);
                swap(T[0],T[i]);
            }
        }
    } else dfs(dfs,0);

    cout << "NO" << ln;
    return 0;
}


Measured memory_bytes: 3628