Constant in a paired-difference variance estimator

Let $\{X_k\}_{k\ge 1}$ be a sequence of i.i.d. random variables with $\mathbb{E}(X_1)=\mu$ and $\mathrm{Var}(X_1)=\sigma^2<\infty$. Suppose $c$ is a constant that does not depend on $n$ such that $$\frac{c}{n}\sum_{i=1}^n(X_{2i}-X_{2i-1})^2$$ is a consistent estimator of $\sigma^2$. Then $c$ is equal to __________ (round off to 2 decimal places)