#include <bits/stdc++.h>
using namespace std;
int n, m;
int a[100100];
void solve() {
  cin >> n >> 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) {
      cout << "NO";
      return;
    }
  }
  if (a[m - 1] == n || a[0] == 1) {
    cout << "NO";
  } else
    cout << "YES";
}
int main() {
  int t = 1;
  while (t--) {
    solve();
  }
  return 0;
}
