Perfect Permutation.cpp

/*A. Perfect Permutation
time limit per test2 seconds
memory limit per test256 megabytes
inputstandard input
outputstandard 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.*/

#include<bits/stdc++.h>
using namespace std;
int main(){
 
   int n;
   cin>>n;
  
   if(n%2!=0){
    cout<<-1<<" ";

   }
   else{
     
     vector<int> perm;

     for(int i=1;i<=n;i++){
        perm.push_back(i);
     }
     for(int i=1;i<perm.size();){
         swap(perm[i], perm[i-1]);
         i+=2;
         if(i<perm.size()){
            continue;
         }
     }

     for(auto const& p : perm){
        cout<<p<<" ";
     }
  
   }


}