#include <iostream>
#include <string>
#include <stack>
#include <sstream>
#include <vector>

using namespace std;

// n - number of tops
// return if sequence is Eilerov path

stack<int> st;
int main()
{
	ios::sync_with_stdio(false);
	string str;
	stringstream ss;

	vector<bool> visited(1000001, false);
	int x;
	getline(cin, str);
	ss << str;
	int temp = -1;
	while (ss >> x) {
		if (st.size() == 0 && temp == -1) {
			temp = x;
			continue;
		}
		if (x == temp) {
			cout << "NO";
			return 0;
		}
		if (st.size() > 0 && x == st.top()) {
			st.pop();
			temp = x;
			continue;
		}
		if (visited[x] == true) {
			cout << "NO";
			return 0;
		}
		visited[temp] = true;
		st.push(temp);
		temp = x;
	}
	if (st.size() > 0) {
		cout << "NO";
	}
	else {
		cout << "YES";
	}
	return 0;
}