Given a Roman numeral, convert it into an integer.

Roman numerals are represented by seven different symbols: I, V, X, L, C, D, M.

Symbol	`I`	`V`	`X`	`L`	`C`	`D`	`M`
Value	1	5	10	50	100	500	1000

For instance, the number 2 is represented as II in Roman numerals, simply two ones added together. Similarly, 13 is written as XIII, which is X + III, and 27 is expressed as XXVII, which breaks down to XX + V + II.

Roman numerals are generally arranged from largest to smallest, moving from left to right. However, instead of writing 4 as IIII, it is represented as IV. Since the I appears before the V, it is subtracted, resulting in four. The same rule applies to 9, which is written as IX.

There are six cases where subtraction is used:

I appears before V (5) and X (10) to form 4 and 9.

X appears before L (50) and C (100) to form 40 and 90.

C appears before D (500) and M (1000) to form 400 and 900.

Example 1:

Input: s = "VI"
Output: 6
Explanation: VI = V + I = 5 + 1 = 6

Example 2:

Input: s = "LVII"
Output: 57
Explanation: LVII = L + V + I + I = 50 + 5 + 1 + 1 = 57

Example 3:

Input: s = "MMCMXCIV"
Output: 2994
Explanation:
	MM = 1000 + 1000 = 2000
	CM = 1000 - 100 = 900 (subtractive notation)
	XC = 100 - 10 = 90 (subtractive notation)
	IV = 5 - 1 = 4 (subtractive notation)
	Total: 2000 + 900 + 90 + 4 = 2994

Example 4:

Input: s = "MMMCMXCIX"
Output: 3999
Explanation:
	MMM = 1000 + 1000 + 1000 = 3000
	CM = 1000 - 100 = 900 (subtractive notation)
	XC = 100 - 10 = 90 (subtractive notation)
	IX = 10 - 1 = 9 (subtractive notation)
	Total: 3000 + 900 + 90 + 9 = 3999

Constraints

1 <= s.length <= 20

s contains only the characters ('I', 'V', 'X', 'L', 'C', 'D', 'M').

It is guaranteed that s is a valid Roman numeral in the range [1, 3999].

32. Roman to integer