An explosion at time t = 0 releases energy 𝐸 at the origin in a space filled with a gas of density σ . Subsequently, a hemispherical blast wave propagates radially outwards as shown in the figure.

Let R denote the radius of the front of the hemispherical blast wave. The radius R follows the relationship 𝑅 = 𝑘 𝑡 ^𝑎𝐸 ^𝑏𝜌 ^𝑐 , where k is a dimensionless constant. The value of exponent a is ___________. (Rounded off to one decimal place)