CodeForces - 233A Perfect Permutation

                                                                             A. Perfect Permutation

                                                                    time limit per test: 2 seconds

                                                              memory limit per test: 256 megabytes

                                                                               input: standard input

                                                                             output: standard output

A permutation is a sequence of integers p1, p2, ..., pn, consisting of n distinct positive integers, each of them doesn't exceed n. Let's denote the i-th element of permutation p as pi. We'll call number n the size of permutation p1, p2, ..., pn.

Nickolas adores permutations. He likes some permutations more than the others. He calls such permutations perfect. A perfect permutation is such permutation p that for any i (1 ≤ i ≤ n) (n is the permutation size) the following equations hold ppi = i and pi ≠ i. Nickolas asks you to print any perfect permutation of size n for the given n.

Input

A single line contains a single integer n (1 ≤ n ≤ 100) — the permutation size.

Output

If a perfect permutation of size n doesn't exist, print a single integer -1. Otherwise print n distinct integers from 1 to n, p1, p2, ..., pn — permutation p, that is perfect. Separate printed numbers by whitespaces.

Examples

input

1

output

-1

input

2

output

2 1 

input

4

output

2 1 4 3 

• 关键是两个公式：ppi = i and pi ≠ i.；把奇偶两位互换即可，当然n得是偶数；

• #include <iostream>
  
  using namespace std;
  
  int main()
  {
      int n;
      cin >> n;
      if( n % 2 == 1 )
      {
          cout << -1 <<endl;
      }
      else
      {
          int i, a[101];
          for( i=1; i<=n; i++ )
          {
              if( i % 2 == 1 )
              {
                  a[i] = i + 1;
              }
              else
              {
                  a[i] = i - 1;
              }
          }
          for( i=1; i<n; i++ )
          {
              cout << a[i] << " ";
          }
          cout << a[i] << endl;
      }
      return 0;
  }