Isoperimetric inequality

Statement

Let $\gamma$ be a simple closed curve in the Euclidean plane ${\mathbb{R}}^2$. Let $Int(\gamma )$ denote the interior of $\gamma$. Let $l(\gamma )$ denote the length of $\gamma$. Then:

$4\pi Ar(Int(\gamma ))\le l(\gamma {)}^2$

where $Ar$ is the area.

Equality holds if and only if $\gamma$ is a circle.