Given a string, s, representing a Roman numeral, return the integer value of the Roman numeral.

Seven different symbols represent Roman numerals:

So, in Roman numerals, $2$ is written as $\text {II}$, which is simply two $1$s added together. The number$12$ is written as $\text{XII}$, which is simply $\text{X} + \text{II}$. The number $27$ is written as $\text{XXVII}$, which breaks down into $\text{XX} + \text{V} + \text{II}$.

Roman numerals are usually written from largest to smallest from left to right ($\text{X}$) comes first, and then $\text I$ for $12$). However, there are six cases where a smaller numeral is placed before a larger one to indicate subtraction. For example, the number $4$ is written as $\text{IV}$, because ...