用java遞歸算法，求1+2+4+8+~~~~~的和？

public class Test4 {

public static void main(String[]args){

System.out.println(num(10));

}

public static int f(int x){

if(x == 1)return 1;

return f(x - 1)*2;

}

public static int num(int x){

if(x == 1)return f(1);

return num(x - 1)+f(x);

}

}