#include <bits/stdc++.h>
using namespace std;
int main() {
  long long int n, m;
  cin >> n >> m;
  if (m == 0) {
    cout << "YES" << endl;
    return 0;
  }
  vector<long long int> v(m);
  for (long long int i = 0; i < m; cin >> v[i++])
    ;
  sort(v.begin(), v.end());
  if (v[0] == 1 || v[m - 1] == n) {
    cout << "NO" << endl;
    return 0;
  }
  int ind = 1, ans = 0;
  for (int i = 0; i < (m - 2); i++) {
    if ((v[i] == (v[i + 1] - 1)) && (v[i + 1] == (v[i + 2] - 1))) {
      cout << "NO" << endl;
      return 0;
    }
  }
  cout << "YES" << endl;
}
