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}