#include <bits/stdc++.h>
using namespace std;
void solve() {
  long long n, k;
  cin >> n >> k;
  vector<long long> v(k);
  for (long long i = 0; i < k; i++) cin >> v[i];
  sort(v.begin(), v.end());
  if (k == 0) {
    cout << "YES" << endl;
    return;
  }
  if (v[0] == 1 || v[k - 1] == n) {
    cout << "NO" << endl;
    return;
  }
  for (long long i = 0; i < k - 2; i++) {
    if (v[i + 2] - v[i] == 2) {
      cout << "NO" << endl;
      return;
    }
  }
  cout << "YES" << endl;
  return;
}
int main() {
  ios_base::sync_with_stdio(false), cin.tie(NULL);
  solve();
  return 0;
}
