formula

${\rm V}[R_1 R_2 \cdots R_N] &=& (\sigma^2 \mu^2)^N - \mu^{2N} \\ &=& \sum_{i=0}^N {}_N C_i \sigma^{2i}\mu^{2(N-i)} - \mu^{2N} \\ &=& \sum_{i=1}^N {}_N C_i \sigma^{2i}\mu^{2(N-i)}$