func_code_string: #include <bits/stdc++.h>   bool isvowel(char s) {     if (s ==  a  || s ==  A  || s ==  e  || s ==  E  || s ==  i  || s ==  I  ||         s ==  o  || s ==  O  || s ==  u  || s ==  U )       return true;     else       return false;   }   long long fact(long long n) {     long long mul = 1;     for (long long i = 1; i <= n; i++) mul = mul * i;     return mul;   }   long long gcd(long long a, long long b) {     return b == 0 ? a : gcd(b % 1000000007, (a % b) % 1000000007);   }   using namespace std;   int main() {     int n;     cin >> n;     if (n & 1)       cout <<  -1             <<   n ;     else {       for (int i = 1; i <= n; i += 2) {         cout << i + 1 <<     << i <<    ;       }     }     return 0;   }   