Repaint for a Dark Stretch

Easy

The tiles of a path are given by the string tiles: 'B' is a black tile and 'W' is a white one. One operation repaints any white tile black. What is the minimum number of operations needed so that the path contains run black tiles in a row somewhere?

Example 1

tiles="BWBBWWB", run=4 → 1

The stretch "BWBB" has one white tile; repaint it and you have four black tiles in a row.

Example 2

tiles="WWBW", run=1 → 0

There is already a black tile, so nothing needs repainting.

Example 3

tiles="WWW", run=3 → 3

Constraints

1 ≤ len(tiles) ≤ 100
Every character of tiles is 'W' or 'B'
1 ≤ run ≤ len(tiles)

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Topics

String Sliding Window

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Input

tiles="BWBBWWB", run=4

Expected

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