#include <bits/stdc++.h>
using namespace std;
int MAX = 1000000;
void printArray(int N, int arr[]) {
  for (int n = 0; n < N; n++) printf("%d ", arr[n]);
  printf("\n");
}
void printVector(vector<int> &v) {
  for (int n = 0; n < v.size(); n++) printf("%d ", v[n]);
  printf("\n");
}
int maxInArray(int i, int j, int A[]) {
  int maxVal = -1e9;
  for (int k = i; k <= j; k++) {
    maxVal = max(maxVal, A[k]);
  }
  return maxVal;
}
int minInArray(int i, int j, int A[]) {
  int minVal = -1e9;
  for (int k = i; k <= j; k++) {
    minVal = min(minVal, A[k]);
  }
  return minVal;
}
long long int gcd(long long int a, long long int b) {
  if (a == 0 || b == 0) return a ^ b;
  return gcd(b % a, a);
}
void zAlgorithm(int len, char s[], int *Z) {
  for (int n = 0; n < len; n++) Z[n] = 0;
  Z[0] = len;
  int l = 0, r = 0;
  for (int k = 1; k < len; k++) {
    if (!(k >= r)) {
      int p = k - l;
      if (Z[p] + k < r) {
        Z[k] = Z[p];
      } else {
        l = k;
        while (r < len && s[r] == s[r - k]) {
          r++;
        }
        Z[k] = r - k;
      }
    } else {
      int i = k, j = 0;
      while (i < len && s[i] == s[j]) {
        i++;
        j++;
      }
      l = k;
      r = i;
      Z[k] = j;
    }
  }
}
int findSmallestDivisor(int len, int *pre) {
  int k = 1;
  while (k <= len) {
    bool isMultiple = true;
    for (int i = 0; i < len; i += k) {
      if (pre[i] < k) {
        isMultiple = false;
        break;
      }
    }
    if (isMultiple) return k;
    k++;
  }
}
void seiveErathosthenes(int limit, vector<int> &primes) {
  if (limit < 2) return;
  bool isPrime[limit + 1];
  for (int i = 2; i < limit + 1; i++) isPrime[i] = true;
  for (int i = 2; i < limit + 1; i++) {
    if (isPrime[i]) {
      primes.push_back(i);
      for (int j = i * 2; j <= limit; j += i) {
        isPrime[j] = false;
      }
    }
  }
}
int main(int argc, char const *argv[]) {
  int N = 0;
  scanf("%d", &N);
  printf("%d\n", N + N / 2);
  for (int n = 0; n < N; n++) {
    if (n % 2) {
      printf("%d ", n + 1);
    }
  }
  for (int n = 0; n < N; n++) {
    if (n % 2 == 0) {
      printf("%d ", n + 1);
    }
  }
  for (int n = 0; n < N; n++) {
    if (n % 2) {
      printf("%d ", n + 1);
    }
  }
  printf("\n");
  return 0;
}
