#include <bits/stdc++.h>
using namespace std;
long long int mpower(long long int x, long long int y, long long int p) {
  long long int res = 1;
  x = x % p;
  while (y > 0) {
    if (y & 1) res = (res * x) % p;
    y = y >> 1;
    x = (x * x) % p;
  }
  return res;
}
long long int power(long long int x, long long int y) {
  long long int res = 1;
  while (y > 0) {
    if (y & 1) res = res * x;
    y = y >> 1;
    x = x * x;
  }
  return res;
}
bool isPrime(long long int n) {
  if (n <= 1) return false;
  if (n <= 3) return true;
  if (n % 2 == 0 || n % 3 == 0) return false;
  long long int p = sqrt(n);
  for (long long int i = 5; i <= p; i = i + 6)
    if (n % i == 0 || n % (i + 2) == 0) return false;
  return true;
}
long long int a[3001];
long long int k;
long long int n;
int32_t main() {
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
  ;
  cin >> n >> k;
  for (long long int i = 0; i < k; i++) cin >> a[i];
  sort(a, a + k);
  long long int curr = 1;
  bool isok = true;
  if (a[0] == 1 || a[k - 1] == n) isok = false;
  for (long long int i = 0; i < k - 2; i++) {
    if (a[i + 2] - a[i] == 2) isok = false;
  }
  if (isok)
    cout << "YES";
  else
    cout << "NO";
  return 0;
}
