#153EasyArrays & Hashing· Frequency Counting

First Unique Keystroke

A typing-practice app records every key a student presses during a drill, using lowercase letters only. To point out the first "one-off" slip, the app wants the position of the earliest keystroke whose letter was pressed **exactly once** in the whole session. Given the keystroke log `s`, return the 0-based index of that keystroke, or `-1` if every letter that appears was pressed at least twice.

Input format

`s`: a string of lowercase English letters.

Output format

An integer: the index of the first character that occurs exactly once in `s`, or -1 if there is none.

Examples

Input: s = "swiss"

Output: 1

s appears three times, w once, i once. The first letter that occurs exactly once is w at index 1.

Input: s = "aabb"

Output: -1

Both letters repeat, so no keystroke is unique.

Input: s = "abcabcd"

Output: 6

a, b and c each appear twice; d appears once and sits at index 6.

Constraints

1 <= s.length <= 10^5
s consists of lowercase English letters (a-z).

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