#include <bits/stdc++.h>
using namespace ::std;
int main() {
  int a[10];
  int n;
  int m2 = -1;
  int ni = 1234123;
  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";
      return 0;
    }
  cout << "YES";
}
