Statistical Inference — unbiased estimator of circle area

The radius of a circle is measured with an error of measurement, which is $N(0,\sigma^2)$. Let $x_1,x_2,…,x_n$ be $n$ measurements of the radius. Let $\bar x=\frac{\sum_{i=1}^n x_i}{n}$ be the sample mean and $S^2=\frac{\sum_{i=1}^n(x_i-\bar x)^2}{n-1}$ be the sample variance. What is the unbiased estimate of the area of the circle?