Given a string S containing lowercase English alphabets. Initially, you are at index 1. You can only jump to some other location j (j>i) only if s[j] = s[i] + 1. Find the maximum index you can reach.
Note: First character of the string is always 'a'. He can only go 'a'->'b','b'->'c','c'->'d' and so on. 'a'->'c','b'->'d','b'->'a' is not a valid move. He can start from any of index of 'a'. Indexing is zero based.

Example 1:
Input:
s = "aaabbcdbdt"
Output:
8
Explanation:
One can start from 'a' at index 0 or 1
or 2 and then move to 'b' at index 3 or
4, then jump to 'c' at index 5 and then
go to 'd' at greater index than 6 which
is 8. Thus the answer is 8.

Example 2:
Input:
s = "aabcb"
Output:
4
Explanation:
Again one can starts from 'a' and proceed
to 'b' and then 'c'. But we have 'b' at
the highest index also, so he will not go
to 'c' and from 'a', we will directly jump
to 'b'.

Your Task:
You don't need to read or print anything. Your task is to complete the function maximum_index() which takes the string S as an input parameter and returns the maximum index where one can reach.

Expected Time Complexity: O(|S|)
Expected Space Complexity: O(1)

Constraints:
1 <= size of string <= 100000

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