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