Area of Figures

<?php

$figure = readline();

if ($figure == "square"){
    $a = readline();
    echo round($a * $a, 3);
}elseif ($figure == "rectangle"){
    $a = readline();
    $b = readline();
    echo round($a * $b, 3);
}elseif ($figure == "circle"){
    $a = readline();
    echo round(pi() * $a * $a, 3);
}elseif ($figure == "triangle"){
    $a = readline();
    $b = readline();
    echo round($a * $b / 2, 3);
}