#include <stdio.h>
#include <atcoder/all>
#include <bits/stdc++.h>
using namespace std;
using namespace atcoder;
using mint = modint998244353;
#define rep(i,n) for (int i = 0; i < (n); ++i)
#define Inf32 1000000001
#define Inf64 4000000000000000001


int main(){
	cin.tie(nullptr);
    ios::sync_with_stdio(false);
	int n,m;
	cin>>n>>m;
	vector<string> s(m);
	rep(i,m)cin>>s[i];
	
	vector<string> t;
	rep(i,n){
		string tt = "";
		rep(j,m)tt += s[j][i];
		t.push_back(tt);
	}
	sort(t.begin(),t.end());
	t.erase(unique(t.begin(),t.end()),t.end());
	if(t.size()==n)cout<<"Yes"<<endl;
	else cout<<"No"<<endl;
	
	return 0;
}