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