Vista - math with "GB/MB/KB" in variables fails, without variables works?

I'm making a script to calculate time for a file to download, I want to be
able to input things like "10GB" and "5GB" and "3.3KB" in the following
script:

$totalsize=read-host "Input file size (gb,mb,kb suffixes allowed)"
$dledsize=read-host "Input Number of downloaded bytes (gb,mb,kb suffixes
allowed)"
$dlspeed=read-host "Input download speed (gb,mb,kb suffixes allowed)"

$time=($totalsize-$dledsize)/$dlspeed/60/60/24

write-host [string]($time) + " days until file is downloaded."

but it gives me an error that says:

"Method invocation failed because [System.String] doesn't contain a method
named 'op_Subtraction'.
At line:1 char:19
+ $time=($totalsize- <<<< $dledsize)/$dlspeed/60/60/24"

but when I try the script in the shell, I can do things like
"$totalsize-5GB" and get the correct result. But I can't use two variables
with the size suffixes. Is this by design or a bug? And is there a (simple)
way around it?

On Nov 1, 12:27*pm, "James R. Gentile" <n...@xxxxxx> wrote:James,

Use the Invoke-Expression Cmdlet on each of the input values to
evaluate them to their numerical equivalents:

$value = Invoke-Expression ( Read-Host "Input file size..." )

Jeff

On Nov 1, 2:48*am, Jeff <jeff.hill...@xxxxxx> wrote:If you do that you'll definitely want to run the input through a regex
before running Invoke-Expression, otherwise a user could enter a
command as their input and it would be executed:

$totalsize=read-host "Input file size (gb,mb,kb suffixes allowed)"

if ($totalsize -match '^\d+[GKM]B$') {
$totalsize = Invoke-Expression $totalsize
} else {
Write-Error "I don't think so."
}