#include <bits/stdc++.h>

using namespace std;

#define int long long
// #define double long double
#define FOR(i, a, b) for(ll i = (a); i < (b); ++i)
#define FORR(i, a, b) for(ll i = (a); i > (b); --i)
#define REP(i, n) for(ll i = 0; i < (n); ++i)
#define REPR(i, n) for(ll i = n; i >= 0; i--)
#define FOREACH(x, a) for(auto &(x) : (a))
#define VECCIN(x)                                                              \
    for(auto &youso_ : (x)) cin >> youso_
#define bitcnt(x) __builtin_popcount(x)
#define lbit(x) __builtin_ffsll(x)
#define rbit(x) __builtin_clzll(x)
#define SZ(x) ((ll)(x).size())
#define fi first
#define se second
#define All(a) (a).begin(), (a).end()
#define rAll(a) (a).rbegin(), (a).rend()

template <typename T = long long> inline T IN() {
    T x;
    cin >> x;
    return (x);
}
inline void CIN() {}
template <class Head, class... Tail>
inline void CIN(Head &&head, Tail &&... tail) {
    cin >> head;
    CIN(move(tail)...);
}
#define CCIN(...)                                                              \
    char __VA_ARGS__;                                                          \
    CIN(__VA_ARGS__)
#define DCIN(...)                                                              \
    double __VA_ARGS__;                                                        \
    CIN(__VA_ARGS__)
#define LCIN(...)                                                              \
    ll __VA_ARGS__;                                                            \
    CIN(__VA_ARGS__)
#define SCIN(...)                                                              \
    string __VA_ARGS__;                                                        \
    CIN(__VA_ARGS__)
#define Yes(a) cout << (a ? "Yes" : "No") << "\n"
#define YES(a) cout << (a ? "YES" : "NO") << "\n"
#define Printv(v)                                                              \
    {                                                                          \
        FOREACH(x, v) { cout << x << " "; }                                    \
        cout << "\n";                                                          \
    }
template <typename T = string> inline void eputs(T s) {
    cout << s << "\n";
    exit(0);
}
template <typename A, size_t N, typename T>
void Fill(A (&array)[N], const T &val) {
    std::fill((T *)array, (T *)(array + N), val);
}

template <typename T> using PQG = priority_queue<T, vector<T>, greater<T>>;
template <typename T> using PQ = priority_queue<T>;

typedef long long ll;
typedef vector<ll> VL;
typedef vector<VL> VVL;
typedef pair<ll, ll> PL;
typedef vector<PL> VPL;
typedef vector<bool> VB;
typedef vector<double> VD;
typedef vector<string> VS;

const int INF = 1e9;
const int MOD = 1e9 + 7;
// const int MOD = 998244353;
const ll LINF = 1e18;
// const double PI = atan(1.0) * 4.0;
const ll dx[] = {1, 1, 0, -1, -1, -1, 0, 1};
const ll dy[] = {0, 1, 1, 1, 0, -1, -1, -1};
#define PI 3.141592653589793238

string S[13];
ll H, W;
ll lp[13];

bool check();
bool dfs(ll);
bool addline();

bool center(ll w, VS b) {
    REP(i, H) if(b[i][w] != b[H - 1 - i][w]) return false;
    return true;
}

bool check() {
    VS b(H);
    VB bused(H);
    REP(i, H) {
        if(bused[lp[i]]) {
            b[H - 1 - lp[i]] = S[i];
        } else {
            b[lp[i]] = S[i];
            bused[lp[i]] = true;
        }
    }
    VL wp(W, -1);
    bool odd = W % 2;
    REP(i, W) {
        if(wp[i] != -1) continue;
        wp[i] = i;
        bool ok = false;
        REP(ii, W) {
            if(wp[ii] != -1) continue;

            ok = true;
            REP(h, H) {
                if(b[h][i] != b[H - 1 - h][ii]){
                    ok = false;
                    break;
                }
            }
            if(ok) {
                wp[ii] = i;
                break;
            }
        }
        if(!ok) {
            if(odd) {
                if(!center(i, b)) return false;
                odd = false;
            } else{
                return false;
            }
        }
    }
    return true;
}

bool addline() {
    H++;
    REP(i, H - 1) {
        S[H - 1] = S[i];
        if(dfs(0)) return true;
    }
    return false;
}

bool dfs(ll pnum) {
    if(H % 2) return addline();
    if(pnum == H / 2) return check();
    ll idx1 = -1;
    REP(i, H) {
        if(lp[i] != -1) continue;
        lp[i] = pnum;
        idx1 = i;
        break;
    }
    REP(i, H) {
        if(lp[i] != -1) continue;
        lp[i] = pnum;
        if(dfs(pnum + 1)) return true;
        lp[i] = -1;
    }
    lp[idx1] = -1;
    return false;
}

signed main() {
    cin >> H >> W;
    REP(i, H) cin >> S[i];
    Fill(lp, -1);
    cout << (dfs(0) ? "YES" : "NO") << "\n";
}


Measured memory_bytes: 256