/**
 * code generated by JHelper
 * More info: https://github.com/AlexeyDmitriev/JHelper
 * @author zjsdut
 */

#include <bitset>
#include <iostream>
using std::to_string;
std::string to_string(const std::string &s) { return '"' + s + '"'; }

std::string to_string(const char *s) { return to_string((std::string) s); }

std::string to_string(bool b) { return (b ? "true" : "false"); }


template<size_t N>
std::string to_string(const std::bitset<N> &bs) {
  return bs.to_string();
}
// the declaration must goes before the definition!
template<typename A>
std::string to_string(const A &v);

template<typename A, typename B>
std::string to_string(const std::pair<A, B> &p) {
  return "(" + to_string(p.first) + ", " + to_string(p.second) + ")";
}

template<typename A>
std::string to_string(const A &v) {
  bool first = true;
  std::string res = "{";
  for (const auto &x : v) {
    if (!first) { res += ", "; }
    first = false;
    res += to_string(x);
  }
  res += "}";
  return res;
}



void debug_out() { std::cerr << std::endl; }

template<typename Head, typename... Tail>
void debug_out(const Head &H, const Tail &... T) {
  std::cerr << " " << to_string(H);
  debug_out(T...);
}

#if defined LOCAL
#define debug(...)                                                                                 \
  std::cerr << "\033[90m" << "[" << #__VA_ARGS__ << "]:", debug_out(__VA_ARGS__),              \
      std::cerr << "\033[0m"
#else
#define debug(...) 42
#endif

#ifndef JHELPER_EXAMPLE_PROJECT_LIBRARY_IO_HPP_
#define JHELPER_EXAMPLE_PROJECT_LIBRARY_IO_HPP_

#include <iomanip>
#include <vector>
struct fast_ios {
  fast_ios() {
    std::cin.tie(nullptr);
    std::ios::sync_with_stdio(false);
    std::cout << std::fixed << std::setprecision(10);
  };
} fast_ios_;

template<typename T>
std::istream &operator>>(std::istream &stream, std::vector<T> &vec) {
  for (auto &x : vec)
    stream >> x;
  return stream;
}

template<typename T, typename U>
std::istream &operator>>(std::istream &in, std::pair<T, U> &p) {
  in >> p.first >> p.second;
  return in;
}

void scan() {}

template<class T, class... Args>
void scan(T &a, Args &... rest) {
  std::cin >> a;
  scan(rest...);
}

template<typename T>
std::ostream &operator<<(std::ostream &stream, const std::vector<T> &vec) {
  bool first = true;
  for (const T &t : vec) {
    if (first) first = false;
    else std::cout << ' ';
    std::cout << t;
  }
  return stream;
}

template<typename T, typename U>
std::ostream &operator<<(std::ostream &out, const std::pair<T,U> &p) {
  out << p.first << ' ' << p.second;
  return out;
}

template<typename T>
void print(const std::vector<std::vector<T>> &t) {
  for (const auto &row : t) {
    std::cout << row << '\n';
  }
}

template<typename T>
void print(const T &t) { std::cout << t << ' '; }

template<typename T, typename... Args>
void print(const T &t, const Args &... rest) {
  print(t);
  print(rest...);
}

template<typename T>
void println(const T &t) { std::cout << t << '\n'; }

template<typename T, typename... Args>
void println(const T &t, const Args &... rest) {
  print(t);
  println(rest...);
}
#endif //JHELPER_EXAMPLE_PROJECT_LIBRARY_IO_HPP_

#ifndef ALGO_UTILS
#define ALGO_UTILS
#include <algorithm>
#include <cassert>
#include <climits>
#include <map>
#include <numeric>
#include <queue>
#include <set>

template<typename A, typename B>
bool chkmin(A &a, const B &b) {
  if (b < a) {
    a = b;
    return true;
  }
  return false;
}

template<typename A, typename B>
bool chkmax(A &a, const B &b) {
  if (b > a) {
    a = b;
    return true;
  }
  return false;
}

using ll = long long;
using ull = unsigned long long;
using vl = std::vector<ll>;
using vb = std::vector<bool>;
using vi = std::vector<int>;
using pii = std::pair<int, int>;
using pli = std::pair<ll, int>;
using pil = std::pair<int, ll>;
using pll = std::pair<ll, ll>;
using vpii = std::vector<pii>;
template<typename T>
class vv {
  std::vector<std::vector<T>> data;

 public:
  vv(int n, int m, T init = T{}) {
    assert(n > 0 && m > 0);
    data.assign(n, std::vector<T>(m, init));
  }
  std::vector<T> &operator[](size_t i) { return data[i]; }
};
#define rng3(i, a, b) for (std::common_type<decltype(a), decltype(b)>::type i = a; i < b; ++i)
#define rng2(i, n) rng3(i, 0, n)
#define GET_MACRO(_1, _2, _3, NAME, ...) NAME
#define rng(...) GET_MACRO(__VA_ARGS__, rng3, rng2)(__VA_ARGS__)
#define up(i, a, b) for (std::common_type<decltype(a), decltype(b)>::type i = a; i <= b; ++i)
#define down(i, b, a) for (std::common_type<decltype(a), decltype(b)>::type i = b; i >= a; i--)
#define TOKENPASTE(x, y) x##y
#define TOKENPASTE2(x, y) TOKENPASTE(x, y)
#define rep(n)                                                                                     \
  for (int TOKENPASTE2(_iter_, __LINE__) = 0, TOKENPASTE2(_num_, __LINE__) = n;                    \
       TOKENPASTE2(_iter_, __LINE__) < TOKENPASTE2(_num_, __LINE__);                               \
       ++TOKENPASTE2(_iter_, __LINE__))
#define FOR(x, cont) for (const auto &x : cont)
#define For(x, cont) for (auto &x : cont)
#define ALL(x) begin(x), end(x)
#define pb push_back
#define mp make_pair
#define eb emplace_back
#define ep emplace
#define SZ(x) (int) (x).size()
#define UNIQ(vec) (vec).erase(std::unique(ALL(vec)), std::end(vec))
#define LB(cont, x) int(std::lower_bound(ALL(cont), x) - std::begin(cont))
#define UB(cont, x) int(std::upper_bound(ALL(cont), x) - std::begin(cont))
template<typename T, typename U = std::less<T>>
using pq = std::priority_queue<T, std::vector<T>, U>;
template<typename T>
struct reversion_wrapper {
  T &iterable;
};
template<typename T>
auto begin(reversion_wrapper<T> w) {
  return std::rbegin(w.iterable);
}
template<typename T>
auto end(reversion_wrapper<T> w) {
  return std::rend(w.iterable);
}
template<typename T>
reversion_wrapper<T> reverse(T &&iterable) {
  return {iterable};
}
template<typename T, typename U>
T ceil(T x, U y) {
  assert(y > 0);
  if (x > 0) x += y - 1;
  return x / y;
}

template<typename T, typename U>
T floor(T x, U y) {
  assert(y > 0);
  if (x < 0) x -= y - 1;
  return x / y;
}
#endif
using namespace std;
class ESymmetricGrid {
 public:
  static void solve(istream &, ostream &) {
    int H, W;
    scan(H, W);
    vector<string> s(H);
    For(row, s) scan(row);
    // permutation of rows
    vector<string> row(H);

    auto check_cols = [&](int i, int j) {
      rng (k, 0, H) {
        if (row[k][i] != row[H - k - 1][j]) return false;
      }
      return true;
    };
    auto is_palindrome = [&](int i) {
      rng (k, 0, H / 2) {
        if (row[k][i] != row[H - k - 1][i]) {
          return false;
        }
      }
      return true;
    };

    auto check = [&]() {
//      debug(row);
      vb used(W);
      bool seen_palindrome = false;
      rng (i, 0, W) {
        if (used[i]) continue;
        bool found = false;
        rng (j, i + 1, W) {
          if (!used[j] && check_cols(i, j)) {
            found = true;
            used[j] = true;
            break;
          }
        }
        if (found) {
          continue;
        }
        if (W & 1 and !seen_palindrome and is_palindrome(i)) {
          seen_palindrome = true;
          continue;
        }
        return false;
      }
      return true;
    };

    vi used(H);
    function<bool(int)> gen_pairing = [&](int cnt) {
      if (cnt == H / 2 + 1) {
        return check();
      }
      rng(i, 0, H) {
        if (!used[i]) {
          row[cnt - 1] = s[i];
          used[i] = true;
          rng(j, i + 1, H) {
            if (!used[j]) {
              used[j] = true;
              row[H - cnt] = s[j];
              if (gen_pairing(cnt + 1)) return true;
              used[j] = false;
            }
          }
          used[i] = false;
          break;
        }
      }
      return false;
    };
    if (H & 1) {
      rng (i, 0, H) {
        used[i] = true;
        row[H / 2] = s[i];
        if (gen_pairing(1)) {
          println("YES");
          return;
        }
        used[i] = false;
      }
      println("NO");
    } else {
      println(gen_pairing(1) ? "YES" : "NO");
    }
  }
};
/*
 * Make sure that your laptop is plugged in or the build may be slow.
 */


int main() {
  ESymmetricGrid solver;
  auto& in = std::cin;
  auto& out = std::cout;
  solver.solve(in, out);
  return 0;
}


Measured memory_bytes: 3644