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


#ifndef SOLUTION_COMMON_H

#include <bits/stdc++.h>

using namespace std;

using ll = long long;
using PI = pair<int, int>;
template<class T> using V = vector<T>;
using VI = V<int>;
#define _1 first
#define _2 second

#ifdef MY_DEBUG
# define DEBUG(x) x
#else
# define DEBUG(x)
#endif

template<class A, class B>
std::ostream & operator <<(ostream &os, const pair<A, B> &p) {
  os << "(" << p._1 << "," << p._2 << ")";
  return os;
}

template<class T>
inline void debug(T &A) {
  DEBUG(
      for (const auto &a : A) {
        cerr << a << " ";
      }
      cerr << '\n';
  )
}

template<class T, class Func>
inline void debug_with_format(T &A, Func f) {
  DEBUG(
      for (const auto &a : A) {
        cerr << f(a) << " ";
      }
      cerr << '\n';
  )
}

template<class T>
inline void debug_dim2(T &A) {
  DEBUG(
      for (const auto &as : A) {
        debug(as);
      }
  )
}

template<typename ... Args>
inline void debug(const char *format, Args const &... args) {
  DEBUG(
      fprintf(stderr, format, args ...);
      cerr << '\n';
  )
}

template<typename ... Args>
string format(const string &fmt, Args ... args) {
  size_t len = snprintf(nullptr, 0, fmt.c_str(), args ...);
  vector<char> buf(len + 1);
  snprintf(&buf[0], len + 1, fmt.c_str(), args ...);
  return string(&buf[0], &buf[0] + len);
}

template<class T1, class T2>
string fmtP(pair<T1, T2> a) {
  stringstream ss;
  ss << "(" << a._1 << "," << a._2 << ")";
  return ss.str();
}

#define SOLUTION_COMMON_H

#endif //SOLUTION_COMMON_H

const int MOD = 1000000007;

class D {
public:
  void solve(std::istream& in, std::ostream& out) {
    ios::sync_with_stdio(false);
    cin.tie(nullptr);

    int h, w;
    in >> h >> w;
    V<V<char>> g(h, V<char>(w));
    for (int i = 0; i < h; ++i) {
      for (int j = 0; j < w; ++j) {
        in >> g[i][j];
      }
    }
    debug_dim2(g);

    bool row = false; // 白黒が全部いっしょの行があった
    for (int i = 0; i < h; ++i) {
      bool same = true;
      for (int j = 1; j < w; ++j) {
        same &= g[i][j] == g[i][j - 1];
      }
      if (same) row = true;
    }
    bool col = false; // 全部一緒の列があった
    for (int j = 0; j < w; ++j) {
      bool same = true;
      for (int i = 1; i < h; ++i) {
        same &= g[i][j] == g[i - 1][j];
      }
      if (same) col = true;
    }

    debug("%d %d", row, col);

    if (row && h > 1 && col && w > 1) {
      out << "NO";
    } else {
      out << "YES";
    }
  }
};


int main() {
	D solver;
	std::istream& in(std::cin);
	std::ostream& out(std::cout);
	solver.solve(in, out);
	return 0;
}