#include <bits/stdc++.h>
using namespace std;
const long long int pinf = ((long long int)2e18);
const long long int ninf = ((long long int)-2e18);
const long long int mod = 1000000007;
struct custom_hash {
  static uint64_t splitmix64(uint64_t x) {
    x += 0x9e3779b97f4a7c15;
    x = (x ^ (x >> 30)) * 0xbf58476d1ce4e5b9;
    x = (x ^ (x >> 27)) * 0x94d049bb133111eb;
    return x ^ (x >> 31);
  }
  size_t operator()(uint64_t x) const {
    static const uint64_t FIXED_RANDOM =
        chrono::steady_clock::now().time_since_epoch().count();
    return splitmix64(x + FIXED_RANDOM);
  }
};
signed main() {
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
  {
    long long int change = 0, m, min_ops = INT_MAX, c, q, W, max_sum = 0,
                  temp_sum = 0, even = 0, odd = 0, color = 1, oka = 5, okb = 0,
                  inda = -1, indb = -1, n, mi = INT_MAX, maxf = -LONG_MAX, y,
                  index = 0, d, z, p = -1, i, j, k, x = 1, flag = 0, l, sum = 0,
                  ans = -1, count5 = 0, count2 = 0, count = 0, count3 = 0, ans1,
                  ans2, min_ind = -1, zero = 0, one = 0;
    cin >> n;
    cout << 3 * n / 2 << '\n';
    for (i = 2; i <= n; i += 2) {
      cout << i << " ";
    }
    for (i = 1; i <= n; i += 2) {
      cout << i << " ";
    }
    for (i = 2; i <= n; i += 2) {
      cout << i << " ";
    }
  }
  return 0;
}
