Stitch multiple video-files together in batches of n

Stitch.ps1

param([int]$episodes_per_file)
$file_count = ( Get-ChildItem .\input | Measure-Object).Count;
$batches = $file_count / $episodes_per_file;
$files = Get-ChildItem .\input |
    Foreach-Object {$_.Name}

for ($i = 0; $i -lt $batches; $i++) {
    $file_text = 'ffmpeg -i "concat:'
    for ($j = 0; $j -lt $episodes_per_file; $j++) {
        $file = $files[$i * $episodes_per_file + $j]
        $file_text += ".\input\$file|"
    }
    $file_text = $file_text.Substring(0, $file_text.Length - 1);
    $file_text += "`"-c:a copy -c:v copy .\output\$i.mp4"
    Invoke-Expression $file_text
}