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If $n$ is an even positive integer, the double factorial notation $n!!$ represents the product of all the even integers from $2$ to $n$. For example, $8!! = 2·4·6·8$. What is the units digit of the following sum?  $2!! + 4!! + 6!! + · · · + 2018!! + 2020!! + 2022!!$ 
Overseas Competition->Knowledge Point->Calculation Modules
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