#include <algorithm>
#include <cmath>
#include <cstdio>
#include <cstdlib>
#include <functional>
#include <iostream>
#include <iterator>
#include <list>
#include <map>
#include <queue>
#include <set>
#include <sstream>
#include <stack>
#include <string>
#include <utility>
#include <valarray>
#include <vector>
#include <limits>

#define N 1000
#define MOD 1000000007
#define pe(str) return cout<<(str)<<endl,0
#define px(str) {cout<<str<<endl;exit(0);}
#define re(i,n) for(int i=0;i<(n);i++)
#define rep(i,a,b) for(int i=(a);i<(b);i++)
#define fe(i,n,f) for_each(i,n,f)
#define bw(a,b,c) (((a)<=(b))&&((b)<=(c)))
using namespace std;

typedef long long ll;
typedef unsigned long long ull;
typedef long double ld;
typedef pair<int,int> pii;

int main(void){
	int n;
	string s;
	std::vector<string> v;
	cin>>n;
	re(i,n)cin>>s,v.push_back(s);
	sort(v.begin(),v.end());
	int i;
	do{
		s="";
		for(i=0;i<n;i++){
			if(s==v[i])break;
			s=v[i];
		}
		if(i==n)pe("YES");
	}while(next_permutation(v.begin(),v.end()));

	pe("NO");
	return 0;
}