#include <bits/stdc++.h>
#define rep(i,a,b) for(int i=a;i<b;i++)

using namespace std;

int main(){
	
	string s[] = {"ne","ushi","tora","u","tatsu","mi","uma","hitsuji","saru,tori","inu","i"}
	, str[50];

	int n;
	cin>>n;
	rep(i,0,n){
		cin>>str[i];
	}

	string  tmp = str[0];
	rep(i,1,n){
		if(str[i] == tmp){
			cout<<"NO"<<endl;
			return 0;
		}
		tmp = str[i];
	}

	cout<<"YES"<<endl;

	
	return 0;
}