#include <bits/stdc++.h>
using namespace std;
template <typename Arg1>
void __f(const char* name, Arg1&& arg1) {
  cerr << name << " : " << arg1 << std::endl;
}
template <typename Arg1, typename... Args>
void __f(const char* names, Arg1&& arg1, Args&&... args) {
  const char* comma = strchr(names + 1, ',');
  cerr.write(names, comma - names) << " : " << arg1 << " | ";
  __f(comma + 1, args...);
}
long long int prod(long long int a, long long int b) {
  long long int x, d;
  if (b == 0)
    return 1;
  else {
    d = prod(a, b / 2);
    x = (d * d) % 1000000007;
    if (b % 2 == 0)
      return x;
    else
      return (x * (a % 1000000007)) % 1000000007;
  }
}
long long int dp[30005][505], d, cnt[30005];
long long int foo(int pos, int step) {
  if (pos > 30000) return 0;
  int stp = step - d + 250;
  if (dp[pos][stp] != -1) return dp[pos][stp];
  long long int res = 0;
  if (step != 1) res = max(res, cnt[pos] + foo(pos + step - 1, step - 1));
  res = max(res, cnt[pos] + foo(pos + step, step));
  res = max(res, cnt[pos] + foo(pos + step + 1, step + 1));
  dp[pos][stp] = res;
  return res;
}
int main() {
  long long int n;
  memset(dp, -1, sizeof(dp));
  scanf("%lld", &n), scanf("%lld", &d);
  for (long long int i = 0; i < n; i++) {
    long long int x;
    scanf("%lld", &x);
    cnt[x]++;
  }
  printf("%lld\n", foo(d, d));
  return 0;
}
