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