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