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