///////////////////////////////////////////////////////////////////////////////

#include <bits/stdc++.h>
#include <unistd.h>
#include <time.h>
#include <sys/time.h>

using namespace std;

///////////////////////////////////////////////////////////////////////////////

#define DEBUG 0

#define pb push_back
#define V vector
#define M unordered_map
#define S static
#define PI  (3.1415926535)

#define rep(i,n) for(ll i=0LL; i<n; ++i)
#define srep(i,s,n) for(ll i=s; i<n; ++i)
#define rrep(i,n) for(ll i=n-1LL; i>=0LL; --i)
#define ALL(a) (a).begin(),(a).end()
#define RALL(a) (a).rbegin(),(a).rend()
#define CIN(x) do { \
        assert(!cin.eof()); \
        cin >> x; \
        assert(!cin.fail()); \
} while(0);
#if DEBUG
#define debug_print(...) _debug_print(__VA_ARGS__)
#define debug_printf(...) printf(__VA_ARGS__)
#define debug_print_time _debug_print_time
#else // DEBUG
#define debug_print(...)
#define debug_printf(...)
#define debug_print_time
#endif // DEBUG

typedef long long ll;
typedef unsigned long long ull;
typedef tuple<ll, ll> t2;
typedef tuple<ll, ll, ll> t3;
typedef tuple<ll, ll, ll, ll> t4;
typedef tuple<ll, ll, ll, ll, ll> t5;
template <typename T>
using priority_queue_incr = priority_queue<T, V<T>, greater<T> >;

///////////////////////////////////////////////////////////////////////////////

template <typename TT, typename T>
T get_m(M<TT, T> &m, TT k, T default_value)
{
    if (m.find(k) == m.end()) return m[k] = default_value;
    return m[k];
}

template <typename TT, typename T>
void incr_m(M<TT, T> &m, TT k)
{
    if (m.find(k) == m.end()) m[k] = 0;
    m[k]++;
}

void llin(ll &a)
{
    CIN(a);
}

void llinl1(auto &v, ll count)
{
    for (ll i = 0LL; i < count; ++i) {
        ll a;
        CIN(a);
        v.push_back(a);
    }
}

void llinl2(auto &v, ll count)
{
    for (ll i = 0LL; i < count; ++i) {
        ll a, b;
        CIN(a >> b);
        v.push_back(t2(a, b));
    }
}

void llinl3(auto &v, ll count)
{
    for (ll i = 0LL; i < count; ++i) {
        ll a, b, c;
        CIN(a >> b >> c);
        v.push_back(t3(a, b, c));
    }
}

void llinl4(auto &v, ll count)
{
    for (ll i = 0LL; i < count; ++i) {
        ll a, b, c, d;
        CIN(a >> b >> c >> d);
        v.push_back(t4(a, b, c, d));
    }
}

void llina(auto &v, ll count)
{
    llinl1(v, count);
}

template <typename T>
void sort(V<T> &v)
{
    sort(v.begin(), v.end());
}

template <typename T>
void sort_reverse(V<T> &v)
{
    sort(v.begin(), v.end(), greater<T>());
}

void _debug_print(auto x)
{
    cout << x << endl;
}

void _debug_print(const t2 &x)
{
    ll x1 = get<0>(x);
    ll x2 = get<1>(x);
    cout << "-- " << x1 << " -- " << x2 << endl;
}

void _debug_print(const t3 &x)
{
    ll x1 = get<0>(x);
    ll x2 = get<1>(x);
    ll x3 = get<2>(x);
    cout << "-- " << x1 << " -- " << x2 << " -- " << x3 << endl;
}

void _debug_print(const t4 &x)
{
    ll x1 = get<0>(x);
    ll x2 = get<1>(x);
    ll x3 = get<2>(x);
    ll x4 = get<3>(x);
    cout << "-- " << x1 << " -- " << x2
         << " -- " << x3 << " -- " << x4 << endl;
}

template <typename T>
void _debug_print(T xarray[], ll n)
{
    rep (i, n) _debug_print(xarray[i]);
}

template <typename T>
void _debug_print(const V<T> &xlist)
{
    for (auto x : xlist) {
        cout << "-- ";
        _debug_print(x);
    }
}

template <typename T>
void _debug_print(const set<T> &xset)
{
    for (auto x : xset) {
        cout << "-- ";
        _debug_print(x);
    }
}

template <typename TT, typename T>
void _debug_print(const M<TT, T> &xlist)
{
    for (auto x : xlist) {
        TT k = x.first;
        T v = x.second;
        cout << "====" << endl;
        cout << "K=";
        _debug_print(k);
        cout << "V=";
        _debug_print(v);
    }
}

void _debug_print_time(const char *prefix)
{
    struct timeval tv;
    gettimeofday(&tv, NULL);
    struct tm *tm = localtime(&tv.tv_sec);
    printf("-- %s %02d:%02d:%02d.%06ld\n",
           prefix, tm->tm_hour, tm->tm_min, tm->tm_sec, tv.tv_usec);
}

///////////////////////////////////////////////////////////////////////////////

#define MOD (1072892182LL)
#define MLT (347289LL)

void _main();
int main()
{
    cout << setprecision(12);
#if !DEBUG
    ios::sync_with_stdio(false);
    cin.tie(0);
#endif

    _main();
    return 0;
}

S ll hashidx;

void f_calc(ll hashlist[12000][2], string resstrlist[12000], const string &s)
{

    ll total = 0;
    rep (i, s.size()) {
        total *= MLT;
        total %= MOD;
        total += s[i];
        total %= MOD;
    }
    hashlist[hashidx][0] = total;
    resstrlist[hashidx] = s;

    total = 0;
    rrep (i, s.size()) {
        total *= MLT;
        total %= MOD;
        total += s[i];
        total %= MOD;
    }
    hashlist[hashidx][1] = total;

    debug_printf("s=%s %s hashidx=%lld (%lld %lld)\n",
                 s.c_str(), resstrlist[hashidx].c_str(),
                 hashidx, hashlist[hashidx][0], hashlist[hashidx][1]);

    hashidx++;
}

void dfs(ll hashlist[12000][2], string resstrlist[12000],
         const string &line, ll w,
         set<ll> &poslist, set<t2> &pairs, char center)
{
    if (poslist.empty()) {
        string s;
        if (center != '_') s += center;
        for (auto p : pairs) {
            ll idx1 = get<0>(p);
            ll idx2 = get<1>(p);
            s = line[idx1] + s + line[idx2];
        }
        f_calc(hashlist, resstrlist, s);
        return;
    }

    ll idx_left = *poslist.begin();
    poslist.erase(idx_left);
    set<ll> lposlist = poslist;

    for (auto idx_right : lposlist) {
        poslist.erase(idx_right);
        pairs.insert(t2(idx_left, idx_right));
        dfs(hashlist, resstrlist, line, w, poslist, pairs, center);
        poslist.insert(idx_right);
        pairs.erase(t2(idx_left, idx_right));
    }

    poslist.insert(idx_left);
}

void f_hashlist(ll hashlist[12000][2], string resstrlist[12000],
                const string &line, ll w)
{
    debug_printf("------------- f_hashlist %s\n", line.c_str());

    hashidx = 0;
    set<ll> poslist;
    rep (i, w) poslist.insert(i);
    set<t2> pairs;

    if (w & 1LL) rep (i, w) {
            poslist.erase(i);
            dfs(hashlist, resstrlist, line, w, poslist, pairs, line[i]);
            poslist.insert(i);
        }
        else {
            dfs(hashlist, resstrlist, line, w, poslist, pairs, '_');
        }
}

bool judge(const ll hashlist[13][12000][2], const string resstrlist[13][12000],
           ll h, ll w, set<ll> &linelist, ll hi)
{
    if (linelist.empty()) return true;
    assert(linelist.size() >= 2);

    ll line1 = *linelist.begin();
    linelist.erase(line1);
    set<ll> locallist = linelist;

    bool ret = false;

    for (auto line2 : locallist) {
        if (hashlist[line1][hi][0] != hashlist[line2][hi][1]) continue;
        linelist.erase(line2);
        bool lf = judge(hashlist, resstrlist, h, w, linelist, hi);
        linelist.insert(line2);
        if (lf) {
            ret = lf;
            break;
        }
    }

    linelist.insert(line1);
    return ret;
}

bool solve(const ll hashlist[13][12000][2], const string resstrlist[13][12000],
           ll h, ll w, ll hashend)
{
    rep (hi, hashend) {
        set<ll> linelist;
        rep (i, h) linelist.insert(i);

        if (h & 1LL) rep (hh, h) {
            if (hashlist[hh][hi][0] != hashlist[hh][hi][1]) continue;
            linelist.erase(hh);
            bool flag = judge(hashlist, resstrlist, h, w, linelist, hi);
            if (flag) return true;
            linelist.insert(hh);
        }
        else {
            bool flag = judge(hashlist, resstrlist, h, w, linelist, hi);
            if (flag) return true;
        }

    }

    return false;
}

void _main()
{
    ll h; llin(h);
    ll w; llin(w);
    S V<string> grid;
    rep (i, h) {
        string tmp; CIN(tmp);
        grid.pb(tmp);
    }

    S ll hashlist[13][12000][2];
    S string resstrlist[13][12000];
    rep (hh, h) f_hashlist(hashlist[hh], resstrlist[hh], grid[hh], w);

    bool flag = solve(hashlist, resstrlist, h, w, hashidx);

    debug_printf("---- ans\n");
    cout << (flag ? "YES": "NO") << endl;
}

///////////////////////////////////////////////////////////////////////////////


Measured memory_bytes: 11264