#include <bits/stdc++.h>
using namespace std;
int n, a[15];
int main() {
  cin >> n;
  for (int i = 0; i < n; i++) cin >> a[i];
  for (int i = 0; i < n - 1; i++)
    if (abs(a[i] - a[i + 1]) >= 2) {
      cout << "NO" << endl;
      return 0;
    }
  cout << "YES" << endl;
  return 0;
}
