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