Given a string S consisting only '0's and '1's, find the last index of the '1' present in it.

Example 1:
Input:
S = 00001
Output:
4
Explanation:
Last index of 1 in given string is 4.

Example 2:
Input:
0
Output:
-1
Explanation:
Since, 1 is not present, so output is -1.

Your Task:
You don't need to read input or print anything. Your task is to complete the function lastIndex() which takes the string S as inputs and returns the last index of '1'. If '1' is not present, return "-1" (without quotes).

Expected Time Complexity: O(N)
Expected Auxiliary Space: O(1)

Constraints:
1 <= |S| <= 10^{6}
S = {0,1}

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