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