Variance of the mean of ten equicorrelated unit-variance variables

Let $\{X_i,i=1,2,\dots,10\}$ be a time series with mean zero, variance one and equicorrelation $1/2$. Let $\bar X_{10}=\tfrac1{10}\sum_{i=1}^{10}X_i$, then variance of $\bar X_{10}$ is: