Consider a polar non-return to zero ($\text{NRZ}$) waveform, using $+2\:V$ and $-2\:V$ for representing binary ‘$1$’ and ‘$0$’ respectively, is transmitted in the presence of additive zero-mean white Gaussian noise with variance $0.4\:V^{2}$. If the a priori probability of transmission of a binary ‘$1$’ is $0.4$, the optimum threshold voltage for a maximum a posteriori ($\text{MAP}$) receiver (rounded off to two decimal places) is ______ $V$.