#include <bits/stdc++.h>
using namespace std;
int main() {
  unsigned long long array[3001], n;
  int m, i, c, f = 0;
  cin >> n;
  scanf("%d", &m);
  if (m == 0) {
    printf("YES");
    return 0;
  }
  for (i = 0; i < m; i++) {
    cin >> array[i];
  }
  sort(array, array + m);
  if (array[m - 1] == n || array[0] == 1) {
    printf("NO");
    return 0;
  }
  c = 0;
  for (i = 0; i < m; i++) {
    if (array[i] + 1 == array[i + 1]) {
      c++;
    } else
      c = 0;
    if (c == 2) {
      f = 1;
      break;
    }
  }
  if (f)
    printf("NO");
  else
    printf("YES");
  return 0;
}
