[R] newbie needs help using R as solver

[Andrej Uduč]

Greetings
I'm a total newbie in R and I'm trying to make a comparisson of Excel 
and R in the fields of:
- optimisation modeling (using solver)
-  decision trees
-  simulation modeling
as described in Winston, Wayne L.:  Practical Management Science.
for optimisation modeling in Excel I would normaly use solver. In R 
however I can't seem to be able to find the solution. I've narrowed it 
down to optim, optimize functions (I might be totaly wrong), but I can't 
figure out how to set the conditions. I've read something about nlm 
model but I can't find the anwser (examples are not easy enough for me).
what I wanna do is solve this simple task:
a+b =< 500000
c+d  =< 500000
g  >= 0,25*c+d
a+b+c+d+g =< 1000000
a, b, c, d, g => 0

I would very much appreciate any help in this matter. I need to locate 
the appropriate function for the task and figure out how to write this 
formulas. I'd also be very thankfull for any help (links) to simple 
examples of decision trees and/or simulation.
   
    Andrej Uduè