two term exponential model

Dear R users,
Does anybody know if there is an R function (package) to fit a two-terms 
exponential model like y = a*exp(bx) + c*exp(dx) where y is dependent 
variable and x is independent variable.  MATLAB has a Curve Fitting Toolbox 
to implement this fitting, but I don't know if there is an R package for 
this fitting.  Thank you!

Deming Mi