# Instructions

Convert a binary number, represented as a string (e.g. '101010'), to its decimal equivalent using first principles.

Implement binary to decimal conversion. Given a binary input
string, your program should produce a decimal output. The
program should handle invalid inputs.

## Note

- Implement the conversion yourself.
  Do not use something else to perform the conversion for you.

## About Binary (Base-2)

Decimal is a base-10 system.

A number 23 in base 10 notation can be understood
as a linear combination of powers of 10:

- The rightmost digit gets multiplied by 10^0 = 1
- The next number gets multiplied by 10^1 = 10
- ...
- The *n*th number gets multiplied by 10^*(n-1)*.
- All these values are summed.

So: `23 => 2*10^1 + 3*10^0 => 2*10 + 3*1 = 23 base 10`

Binary is similar, but uses powers of 2 rather than powers of 10.

So: `101 => 1*2^2 + 0*2^1 + 1*2^0 => 1*4 + 0*2 + 1*1 => 4 + 1 => 5 base 10`.
