Combine analytical solution and numerical solutions.

S_net  = S1 * S2

S1 has a analytic form.  S2 has to be found by simulation.  How do we combine these two in numerical simulation? We can simulate S2 and draw random number from S1.  The network lifespan is the minimal of the two:
   network_lifespan = min(lifespan1, lifespan2).
S_net can then be calculated from the distribution of network_lifespans.