Constant for unbiased normal scale estimation

Let $X_1, X_2, \dots, X_{10}$ be a random sample from an $N(0, \sigma^2)$ distribution, where $\sigma > 0$ is an unknown parameter. For some real constant $c$, let $Y = \frac{c}{10}\sum_{i=1}^{10} |X_i|$ be an unbiased estimator of $\sigma$. Then, the value of $c$ equals __________ (round off to two decimal places)