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

bool check(const std::vector<int>& seq) {
    size_t n = (seq.size() + 1) / 2;
    std::vector<bool> is_leave;
    is_leave.reserve(n);
    for (size_t it = 0; it < n; ++it) {
        is_leave.push_back(false);
    }

    std::vector<int> count;
    count.reserve(n);
    for (size_t it = 0; it < n; ++it) {
        count.push_back(0);
    }

    for (auto elem : seq) {
        ++count[elem];
    }

    for (size_t it = 0; it < n; ++it) {
        if (count[it] == 1) {
            is_leave[it] = true;
        }
    }

    std::stack<int> stack;
    for (auto elem : seq) {
        if (is_leave[elem]) {
            continue;
        }
        if (!stack.empty() && stack.top() == elem) {
            stack.pop();
            continue;
        }
        stack.push(elem);
    }
    return stack.empty();
}

bool Check(const std::vector<int>& seq) {
    size_t n = (seq.size() + 1) / 2;
    std::vector<bool> is_visited;
    std::unordered_set<int> set;
    is_visited.reserve(n);
    for (size_t it = 0; it < n; ++it) {
        is_visited.push_back(false);
    }

    std::stack<int> stack;
    stack.push(0);
    for (auto elem : seq) {
        if (is_visited[elem]) {
            if (stack.top() == elem) {
                return false;
            }
            stack.pop();
            if (stack.top() != elem) {
                return false;
            }
        } else {
            stack.push(elem);
            is_visited[elem] = true;
            set.insert(elem);
        }
    }
    return set.size() == n;
}

std::vector<int> get_numbers(const std::string& str) {
    std::vector<int> res;
    res.reserve(200'000);
    std::stringstream s;
    s << str;
    std::string temp;
    int found;
    while (!s.eof()) {
        s >> temp;
        if (std::stringstream(temp) >> found) {
            res.push_back(found);
        }
    }
    return res;
}

int main() {
    std::ios_base::sync_with_stdio(0);
    std::vector<int> seq;
    std::string str;
    getline(std::cin, str);
    seq = get_numbers(str);
    if (Check(seq)) {
        std::cout << "YES";
    } else {
        std::cout << "NO";
    }
}