#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];
  }
  for (int i = 0; i < n - 1; i++) {
    if (abs(a[i] - a[i + 1]) >= 2) {
      cout << "No" << endl;
      return 0;
    }
  }
  cout << "Yes" << endl;
}
