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