Given `head` which is a reference node to a singly-linked list. The value of each node in the linked list is either `0` or `1`. The linked list holds the binary representation of a number.

Return the decimal value of the number in the linked list.

Example 1:

    Input: head = [1,0,1]
    Output: 5
    Explanation: (101) in base 2 = (5) in base 10

Example 2:

    Input: head = [0]
    Output: 0

Constraints:

- The Linked List is not empty.
- Number of nodes will not exceed `30`.
- Each node's value is either `0` or `1`.