Part 1

[long[]]$inputFile = Get-Content 'C:\Scripts\AdventOfCode 2020\9\input.txt'

Function checkPreamble {
    Param(
        $checkingNumber,
        $preamble,
        [switch]$odd
    )
    $evenNumbers = $preamble -match '^\d*[02468]$'
    $oddNumbers = $preamble -match '^\d*[13579]$'
    if (!$odd){
        For($j = 0; $j -lt $evenNumbers.Count; $j++){
            if($evenNumbers -contains ($checkingNumber - $evenNumbers[$j])){
                return $false
            }
        }

        For($j = 0; $j -lt $oddNumbers.Count; $j++){
            if($oddNumbers -contains ($checkingNumber - $oddNumbers[$j])){
                return $false
            }
        }
    } else {
        For($j = 0; $j -lt $evenNumbers.Count; $j++){
            if($oddNumbers -contains ($checkingNumber - $evenNumbers[$j])){
                return $false
            }
        }
    }
    return $checkingNumber
}

$startIndex = 26

For ($i = $startIndex; $i -lt $inputFile.Count; $i++){
    $preamble = $inputFile[($i -25)..($i -1)]
    switch -Regex ($inputFile[$i]){
        '^\d*[02468]$' {
            $returnedVariable = checkPreamble -checkingNumber $inputFile[$i] -preamble $preamble
            if($returnedVariable -ne $false){
                $contagiousNumber = $returnedVariable
                break
            }
        }
        '^\d*[13579]$' {
            $returnedVariable = checkPreamble -checkingNumber $inputFile[$i] -preamble $preamble -odd
            if($returnedVariable -ne $false){
                $contagiousNumber = $returnedVariable
                break
            }
        }
    }
}

Write-Output $contagiousNumber