#include <bits/stdc++.h>
using namespace std;
int main() {
  int a, b;
  cin >> a >> b;
  vector<int> hi;
  for (int g = 0; g < b; g++) {
    int t;
    cin >> t;
    hi.push_back(t);
    if (t == a || t == 1) {
      cout << "NO";
      return 0;
    }
  }
  if (a == 1) {
    cout << "YES";
    return 0;
  }
  int numberconsecutive = 0;
  sort(hi.begin(), hi.end());
  for (int g = 1; g < hi.size(); g++) {
    if (hi[g] == hi[g - 1] + 1) {
      numberconsecutive += 1;
      if (g == hi.size() - 1) {
        numberconsecutive += 1;
        if (numberconsecutive >= 3) {
          cout << "NO";
          return 0;
        }
      }
    } else {
      numberconsecutive += 1;
      if (numberconsecutive >= 3) {
        cout << "NO";
        return 0;
      }
      numberconsecutive = 0;
    }
  }
  cout << "YES";
  return 0;
}
