Roman numerals are represented by seven different symbols: `I`, `V`, `X`, `L`, `C`, `D` and `M`.
Given a roman numeral, convert it to an integer.
**Example 1:**
```
Input: s = "III"
Output: 3
```
**Example 2:**
```
Input: s = "LVIII"
Output: 58
Explanation: L=50, V=5, III=3
```
**Example 3:**
```
Input: s = "MCMXCIV"
Output: 1994
Explanation: M=1000, CM=900, XC=90, IV=4
```