<p>You are given a <strong>0-indexed</strong> string <code>blocks</code> of length <code>n</code>, where <code>blocks[i]</code> is either <code>&#39;W&#39;</code> or <code>&#39;B&#39;</code>, representing the color of the <code>i<sup>th</sup></code> block. The characters <code>&#39;W&#39;</code> and <code>&#39;B&#39;</code> denote the colors white and black, respectively.</p>

<p>You are also given an integer <code>k</code>, which is the desired number of <strong>consecutive</strong> black blocks.</p>

<p>In one operation, you can <strong>recolor</strong> a white block such that it becomes a black block.</p>

<p>Return<em> the <strong>minimum</strong> number of operations needed such that there is at least <strong>one</strong> occurrence of </em><code>k</code><em> consecutive black blocks.</em></p>

<p>&nbsp;</p>
<p><strong class="example">Example 1:</strong></p>

<pre>
<strong>Input:</strong> blocks = &quot;WBBWWBBWBW&quot;, k = 7
<strong>Output:</strong> 3
<strong>Explanation:</strong>
One way to achieve 7 consecutive black blocks is to recolor the 0th, 3rd, and 4th blocks
so that blocks = &quot;BBBBBBBWBW&quot;. 
It can be shown that there is no way to achieve 7 consecutive black blocks in less than 3 operations.
Therefore, we return 3.
</pre>

<p><strong class="example">Example 2:</strong></p>

<pre>
<strong>Input:</strong> blocks = &quot;WBWBBBW&quot;, k = 2
<strong>Output:</strong> 0
<strong>Explanation:</strong>
No changes need to be made, since 2 consecutive black blocks already exist.
Therefore, we return 0.
</pre>

<p>&nbsp;</p>
<p><strong>Constraints:</strong></p>

<ul>
	<li><code>n == blocks.length</code></li>
	<li><code>1 &lt;= n &lt;= 100</code></li>
	<li><code>blocks[i]</code> is either <code>&#39;W&#39;</code> or <code>&#39;B&#39;</code>.</li>
	<li><code>1 &lt;= k &lt;= n</code></li>
</ul>

class Solution:
    def minimumRecolors(self, blocks: str, k: int) -> int:
        ans = inf
        low = -1
        white = 0
        n = len(blocks)

        for index , block in enumerate(blocks):
            if block =='W':
                white += 1
            if index- low >= k  :
                ans = min(white , ans)
                low += 1
                white -= (blocks[low] == 'W')
        return -1 if ans == inf else ans
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