poj 1286 Necklace of Beads(polya)

Beads of red, blue or green colors are connected together into a circular necklace of n beads ( n < 24 ). If the repetitions that are produced by rotation around the center of the circular necklace or reflection to the axis of
symmetry are all neglected, how many different forms of the necklace are there? 



Input
The input has several lines, and each line contains the input data n. 

-1 denotes the end of the input file. 

Output
The output should contain the output data: Number of different forms, in each line correspondent to the input data.

Sample Input

4
5
-1

Sample Output

21
39

#include <iostream>
#include <cstdio>
#include <cmath>
#include <cstring>
#include <algorithm>
#include <vector>
#include <map>
#include <string>
#include <queue>
#include <string>
using namespace std;
const int N = 1e6+10;
typedef long long LL;
LL gcd(LL a,LL b)
{
return b?gcd(b,a%b):a;
}

LL power(LL x,LL n)
{
LL ans=1;
while(n)
{
if(n&1) ans*=x;
n>>=1;
x*=x;
}
return ans;
}

int main()
{
LL m=3, n;
while(scanf("%lld", &n)!=EOF&&n!=-1)
{
if(n==0)
{
puts("0");
continue;
}
LL cnt=0;
for(int i=1;i<=n;i++) cnt+=power(m,gcd(i,n));
if(n&1) cnt+=(n*power(m,n/2+1));
else cnt+=(n/2)*(power(m,n/2+1)+power(m,n/2));
cnt/=(n*2);
printf("%lld\n",cnt);
}
return 0;
}