#include <bits/stdc++.h>
using namespace std;
int main() {
  long long n, dirty, arr[3010];
  cin >> n >> dirty;
  bool yow = true;
  for (long long i = 0; i < dirty; i++) {
    cin >> arr[i];
  }
  sort(arr, arr + dirty);
  for (long long i = 1; i < dirty; i++) {
    if (arr[i - 1] == arr[i] - 1 && arr[i + 1] == arr[i] + 1) {
      yow = false;
      break;
    }
  }
  if (arr[dirty - 1] == n || arr[0] == 1)
    cout << "NO" << endl;
  else if (yow)
    cout << "YES" << endl;
  else
    cout << "NO" << endl;
  return 0;
}
