#include <bits/stdc++.h>
using namespace std;
int tot, l, n;
int a[1000010];
int read() {
  int pp = 0, ff = 1;
  char ch;
  ch = getchar();
  while (ch < '0' || ch > '9') {
    if (ch == '-') ff = -1;
    ch = getchar();
  }
  while (ch >= '0' && ch <= '9') {
    pp = pp * 10 + ch - '0';
    ch = getchar();
  }
  return pp * ff;
}
int main() {
  n = read();
  for (int i = 2; i <= n; i += 2) a[++tot] = i;
  for (int i = 1; i <= n; i += 2) a[++tot] = i;
  for (int i = 2; i <= n; i += 2) a[++tot] = i;
  printf("%d\n", tot);
  for (int i = 1; i <= tot; i++) printf("%d ", a[i]);
  return 0;
}
