Geometric Mean for a given K Using Recursion

Example:

For a given K find:
                                        1 + 1/2 + 1/4 + 1/8 + ... + 1/(2^k) 





#include <iostream>
#include<math.h>

using namespace std;
///That function should also be of the same dataType as that the answer it is returning
 float Gmean(int k,int x,double double sum)
{
    if(k==x)
    {
        sum=sum+(1/(pow(2,k)));
        return sum;
    }
    sum=sum+(1/(pow(2,k)));
     double double  y=Gmean(k+1,x,sum);
    return y;
}

int main()
{
    int x;
    cin>>x;
    double double  y=Gmean(0,x,0);
    cout<<y;
    return 0;
}