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