powershell变量做减法时被当字符串

powershell 变量问题

#test.ps1
$n = Read-Host 'Get Number'
function  Mtest {
    param (
        $ap
    )
    $ap 
}
Mtest $n-1

输出：
PS C:\Users\user> .\test.ps1
Get Number: 3
3-1

#test.ps1
$n = Read-Host 'Get Number'
function  Mtest {
    param (
        $ap
    )
    $ap 
}
Mtest ($n-1)