RosettaCodeData/Task/Fibonacci-sequence/PowerShell/fibonacci-sequence-1.psh

function fib ($n) {
    if ($n -eq 0) { return 0 }
    if ($n -eq 1) { return 1 }

    $m = 1
    if ($n -lt 0) {
        if ($n % 2 -eq -1) {
            $m = 1
        } else {
            $m = -1
        }

        $n = -$n
    }

    $a = 0
    $b = 1

    for ($i = 1; $i -lt $n; $i++) {
        $c = $a + $b
        $a = $b
        $b = $c
    }

    return $m * $b
}