#include <iostream>
using namespace std;
int pyra(const int *array, int n) {
    if (n >= 3) {
        for (int i = 0; i < n / 2 - 1; i++) {
            if ((array[i] > array[2 * i + 1]) || (array[i] > array[2 * i + 2]))
                return 0;

        }
    }
    return 1;
}
int main() {
    freopen("isheap.in", "r", stdin);
    freopen("isheap.out", "w", stdout);
    char* answer[2] = {"NO", "YES"};
    int n;
    cin>>n;
    int* arr = new int[n];
    for(int i=0; i<n; i++){
        cin>>arr[i];
    }
    int result = pyra(arr, n);
    cout<<answer[result];
    return 0;
}