#include <bits/stdc++.h>

#define FOR(i, a, b) for(int i= (a); i<((int)b); ++i)
#define RFOR(i, a) for(int i=(a); i >= 0; --i)
#define FOE(i, a) for(auto i : a)
#define ALL(c) (c).begin(), (c).end()
#define RALL(c) (c).rbegin(), (c).rend()
#define DUMP(x)  cerr << #x << " = " << (x) << endl;
#define SUM(x) std::accumulate(ALL(x), 0LL)
#define MIN(v) *std::min_element(v.begin(), v.end())
#define MAX(v) *std::max_element(v.begin(), v.end())
#define EXIST(v, x) (std::find(v.begin(), v.end(), x) != v.end())
#define BIT(n) (1LL<<(n))
#define UNIQUE(v) v.erase(unique(v.begin(), v.end()), v.end());

typedef long long LL;
template<typename T> using V = std::vector<T>;
template<typename T> using VV = std::vector<std::vector<T>>;
template<typename T> using VVV = std::vector<std::vector<std::vector<T>>>;

template<class T>
inline T ceil(T a, T b) { return (a + b - 1) / b; }

template<class T>
inline void print(T x) { std::cout << x << std::endl; }

template<class T>
inline void print_vec(const std::vector<T> &v) {
    for (int i = 0; i < v.size(); ++i) {
        if (i != 0) { std::cout << " "; }
        std::cout << v[i];
    }
    std::cout << "\n";
}

template<class T>
inline bool inside(T y, T x, T H, T W) { return 0 <= y and y < H and 0 <= x and x < W; }

template<class T>
inline double euclidean_distance(T y1, T x1, T y2, T x2) {
    return std::sqrt((x1 - x2) * (x1 - x2) + (y1 - y2) * (y1 - y2));
}

template<class T>
inline double manhattan_distance(T y1, T x1, T y2, T x2) { return abs(x1 - x2) + abs(y1 - y2); }

const int INF = 1L << 30;
const double EPS = 1e-9;
const std::string YES = "YES", Yes = "Yes", NO = "NO", No = "No";
const std::vector<int> dy4 = {0, 1, 0, -1}, dx4 = {1, 0, -1, 0};    // 4近傍（右, 下, 左, 上）
const std::vector<int> dy8 = {0, -1, 0, 1, 1, -1, -1, 1}, dx8 = {1, 0, -1, 0, 1, 1, -1, -1};

using namespace std;

bool check(const vector<pair<int, int>> &pv, const vector<string> &field) {
    int H = field.size();
    int W = field[0].size();

    vector<int> v(W, -1);
    vector<bool> used(W, false);

    int pos = 0;
    FOR(i, 0, pv.size()) {
        int x1 = pv[i].first;
        int x2 = pv[i].second;

        if (x1 >= W) {
            v[W / 2] = x2;
            continue;
        }
        else if (x2 >= W) {
            v[W / 2] = x1;
            continue;
        }

        v[pos] = x1;
        used[x1] = true;
        v[v.size() - 1 - pos] = x2;
        used[x2] = true;
        pos++;

    }

//    if (v[0] = 0 and v[1] == 3 and  v[3] == 4 and v[4] == 2) {
//        print_vec(v);
//    }


    vector<int> used2(H, false);
    FOR(y1, 0, H) {
        if (used2[y1]) { continue; }

        FOR(y2, y1 + 1, H) {
            if (used2[y2]) { continue; }

            bool ok = true;
            FOR(x, 0, W) {
                ok &= field[y1][v[x]] == field[y2][v[W - 1 - x]];
            }
            if (ok) {
                used2[y1] = true;
                used2[y2] = true;
                break;
            }
        }
    }

    int num = 0;
    FOR(y, 0, used2.size()) {
        if (not used2[y]) {
            num++;
            FOR(x, 0, W) {
                if (field[y][v[x]] != field[y][v[W - 1 - x]]) {
                    return false;
                }
            }
        }
    }

    if (num >= 2) {
        return false;
    }

    if (H % 2 == 0 and num >= 1) {
        return false;
    }

    return true;

}

bool dfs(int idx, vector<bool> &used, vector<pair<int, int>> &pv, const vector<string> &field) {
    const int W = field[0].size();

    if (idx >= (W + (W % 2 == 1)) / 2) {
//        FOE(p, pv) {
//            cout << p.first << "-" << p.second << ", ";
//        }
//        cout << endl;
        return check(pv, field);
    }

    int i = -1;
    FOR(j, 0, W) {
        if (used[j]) { continue; }
        i = j;
        break;
    }
    used[i] = true;

    bool ok = false;
    FOR(j, 0, W + (W % 2 == 1)) {
        if (used[j]) { continue; }
        used[j] = true;
        pv.emplace_back(make_pair(i, j));

        ok |= dfs(idx + 1, used, pv, field);

        used[j] = false;
        pv.pop_back();
    }
    used[i] = false;

    return ok;
}

int main() {
    cin.tie(0);
    ios::sync_with_stdio(false);

    int H, W;
    cin >> H >> W;
    vector<string> field(H);
    FOR(y, 0, H) {
        string S;
        cin >> S;
        field[y] = S;
    }

    vector<bool> used(15, false);
    vector<pair<int, int>> pv;
    if (dfs(0, used, pv, field)) {
        print(YES);
    }
    else {
        print(NO);
    }

    return 0;
}

Measured memory_bytes: 256