<p>You are given two strings <code>s1</code> and <code>s2</code> of equal length consisting of letters <code>&quot;x&quot;</code> and <code>&quot;y&quot;</code> <strong>only</strong>. Your task is to make these two strings equal to each other. You can swap any two characters that belong to <strong>different</strong> strings, which means: swap <code>s1[i]</code> and <code>s2[j]</code>.</p>

<p>Return the minimum number of swaps required to make <code>s1</code> and <code>s2</code> equal, or return <code>-1</code> if it is impossible to do so.</p>

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

<pre>
<strong>Input:</strong> s1 = &quot;xx&quot;, s2 = &quot;yy&quot;
<strong>Output:</strong> 1
<strong>Explanation:</strong> Swap s1[0] and s2[1], s1 = &quot;yx&quot;, s2 = &quot;yx&quot;.
</pre>

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

<pre>
<strong>Input:</strong> s1 = &quot;xy&quot;, s2 = &quot;yx&quot;
<strong>Output:</strong> 2
<strong>Explanation:</strong> Swap s1[0] and s2[0], s1 = &quot;yy&quot;, s2 = &quot;xx&quot;.
Swap s1[0] and s2[1], s1 = &quot;xy&quot;, s2 = &quot;xy&quot;.
Note that you cannot swap s1[0] and s1[1] to make s1 equal to &quot;yx&quot;, cause we can only swap chars in different strings.
</pre>

<p><strong class="example">Example 3:</strong></p>

<pre>
<strong>Input:</strong> s1 = &quot;xx&quot;, s2 = &quot;xy&quot;
<strong>Output:</strong> -1
</pre>

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

<ul>
	<li><code>1 &lt;= s1.length, s2.length &lt;= 1000</code></li>
	<li><code>s1.length == s2.length</code></li>
	<li><code>s1, s2</code> only contain <code>&#39;x&#39;</code> or <code>&#39;y&#39;</code>.</li>
</ul>

class Solution:
    def minimumSwap(self, s1: str, s2: str) -> int:
        xy = yx = 0
        for c1, c2 in zip(s1, s2):
            if c1 == 'x' and c2 == 'y':
                xy += 1
            elif c1 == 'y' and c2 == 'x':
                yx += 1
        
        if (xy + yx) % 2:
            return -1
        
        return xy // 2 + yx // 2 + (xy % 2) * 2
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