eclipse新手 想知道int和double怎么乘起来啊

if(input==2)
{
System.out.println("YOU GET A SPHERE! Lets try to figure out the volume of cube!!\nPlease input any numbers");
num=keyboard.nextInt();
multiply=
System.out.println("\nYour answer is : " +multiply );
}
就是这里啊 num是 int π是double 我得乘起来怎么写啊QAQQQQ

啊咧 被吞了