using System;
using static System.Console;
using System.Linq;
using System.Collections.Generic;
class Program
{
    static int NN => int.Parse(ReadLine());
    static string[] SList(long n) => Enumerable.Repeat(0, (int)n).Select(_ => ReadLine()).ToArray();
    public static void Main()
    {
        Solve();
    }
    static void Solve()
    {
        var n = NN;
        var m = NN;
        var list = new bool[n * n];
        for (var i = 0; i < n; ++i) list[n * i + i] = true;
        for (var i = 0; i < m; ++i)
        {
            var s = ReadLine();
            for (var j = 0; j < s.Length; ++j) for (var k = j + 1; k < s.Length; ++k)
            {
                if (s[j] != s[k])
                {
                    list[j * n + k] = true;
                    list[k * n + j] = true;
                }
            }
        }
        WriteLine(list.All(f => f) ? "Yes" : "No");
    }
}