#include <iostream>
#include <fstream>
using namespace std;

bool sift_down(int* a, int n, int i) {
    int par = i;
    int l_child = 2 * i + 1;
    int r_child = 2 * i + 2;
    if (l_child < n && a[l_child] < a[par]) par = l_child;
    if (r_child < n && a[r_child] < a[par]) par = r_child;
    if (par != i) return true;
    return false;
}
int main(){
    ifstream fin("isheap.in");
    ofstream fout("isheap.out");
    int n;
    fin >> n;
    int* a = new int[n];
    for (int i = 0; i < n; i++) fin >> a[i];
    for (int i = n / 2 - 1; i >= 0; i--) {
        if (sift_down(a, n, i)) {
            fout << "NO" << endl;
            return 0;
        }
    }
    fout << "YES" << endl;
    return 0;
}