Problem statement

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You are given a string $\text{'}S\text{'}$ of length $\text{'}N\text{'}$ and an integer $\text{'}K\text{'}.\text{'}K\text{'}$ is a divisor of $\text{'}N\text{'}.$

Suppose you break the string into $\text{'}M\text{'}$ subarrays $($where $\text{'}M\text{'}={?}^{\text{'}}N\text{'}/\text{'}$K$\text{'}),$ i$.$e$.,$

The first substring $\text{'}S1\text{'}$ is $\text{'}S[1$dotsK$]\text{'}.$

The second substring $\text{'}S2\text{'}$ is $\text{'}S[K+1$dots$2^{***}K]\text{'}.$

$...$

The $\text{'}M\text{'}$th substring is $\text{'}SM\text{'}$ is $\text{'}*[$:$K\}\text{'}.$

You are allowed to do the following operation:

Choose any index $\text{'}$i$\text{'}$ of the string $\text{'}$S$\text{'}.$

Replace it with any lowercase character. The cost of this operation is $\text{'}1.$:

Your task is to convert the string $\text{'}S\text{'}$ such that:

All the characters in a substring are same for each of the $\text{'}M\text{'}$ substrings.

Any two consecutive substrings do not have the same character.

Your task is to find the minimum cost to do so and return it$.$

Note:

For a string $\text{'}{S}^{\text{'}},S[i...j]$ denotes the string $\text{'}$S$\text{'}$ substring from index $\text{'}i\text{'}$ to index $\text{'}$j$].$