764

#include <iostream>
#include <fstream>
#include <algorithm>

using namespace std;

struct Node {
    Node* l;
    Node* r;
    int val;
    Node(int val) :val(val), l(nullptr), r(nullptr) {};
};

Node* root = nullptr;

void add(int val, Node* cur) {
    if (cur == nullptr) {
        root = new Node(val);
        return;
    }
    if (val < cur->val) {
        if (cur->l == nullptr) {
            cur->l = new Node(val);
            return;
        }
        add(val, cur->l);
    }
    else if (val > cur->val) {
        if (cur->r == nullptr) {
            cur->r = new Node(val);
            return;
        }
        add(val, cur->r);
    }
}

int get_h(Node* cur) {
    if (cur == nullptr) return 0;
    return max(get_h(cur->l), get_h(cur->r)) + 1;
}

bool bal(Node* cur) {
    return get_h(cur) == 0 || (bal(cur->l) && bal(cur->r) && abs(get_h(cur->l) - get_h(cur->r)) <= 1);
}

int main() {
    ifstream fin("input.txt");
    ofstream fout("output.txt");

    int a; fin >> a;
    while (a != 0) {
        add(a, root);
        fin >> a;
    }
    fout << (bal(root) ? "YES" : "NO") << endl;

    fin.close();
    fout.close();
    return 0;
}