#include <bits/stdc++.h>
using namespace std;
int a[200009], n;
int main() {
  cin >> n;
  for (int i = 1; i <= n; i++) cin >> a[i];
  for (int i = 2; i <= n; i++)
    if (a[i] < a[i + 1] && a[i] < a[i - 1]) {
      cout << "NO";
      return 0;
    }
  cout << "YES";
}
