error: cannot find symbol

import java.util.*;
public class GeometricSeries {
public static void main(String [] args)
{
double S,R;
double a;
int n;
Scanner kb = new Scanner(System.in);
System.out.print("a=");
a= kb.nextInt();
System.out.print("R=");
R= kb.nextInt();
System.out.print("n=");
n= kb.nextInt();
double Rn = Math.pow(R, n);
S = a (Rn - 1) / (R - 1);
System.out.println("S=" + S);
}
}
S = a (Rn - 1) / (R - 1);
^
symbol: method a(double)
这应该如何解决?

乘法的话a后加上 * 号

account类是不是没有（double,double)的构造方法呢？