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