Matching format tags

tokens holds, in order, the formatting tags written in a post. "<name>" opens the format named name, and "</name>" closes it. A closing tag must have the same name as the most recently opened tag that is still open. A closing tag that appears when no tag is open also breaks this rule.

Complete the function solution, which returns the position of the first closing tag that breaks the rule. Positions are counted from 0 at the start of tokens. If no tag breaks the rule but some tags are still open at the end, return len(tokens). If all the tags match, return -1.

Function

solution(tokens)

Constraints

• The length of tokens is from 1 to 100,000.
• Each tag is "<name>" or "</name>", where name is made of lowercase English letters, with a length from 1 to 10.

Examples

Starter code

def solution(tokens):
    answer = 0
    return answer