#include <bits/stdc++.h>
using namespace std;
int a[1000];
int main() {
  int n;
  cin >> n;
  for (int i = 1; i <= n; i++) {
    cin >> a[i];
  }
  for (int i = 1; i < n; i++) {
    if (a[i] - a[i + 1] > 1) {
      cout << "NO";
      return 0;
    }
  }
  cout << "YES";
  return 0;
}
