#include <bits/stdc++.h>
using namespace std;
const int N = 1e5 + 9, M = 3e5 + 9, OO = 0x3f3f3f3f;
const double EPS = 0.00000000001;
const int mod = 1000003;
int n, m, d[3005];
bool flage = 1;
int main() {
  scanf("%d%d", &n, &m);
  for (int i = 0; i < m; ++i) {
    scanf("%d", d + i);
    if (d[i] == 1 || d[i] == n) flage = 0;
  }
  sort(d, d + m);
  int mx = min(1, m);
  for (int i = 0, j = 1; j < m; ++j)
    if (d[j] == d[j - 1] + 1)
      mx = max(mx, j - i + 1);
    else
      i = j;
  if (mx >= 3) flage = 0;
  if (flage)
    puts("YES");
  else
    puts("NO");
  return 0;
}
