solving system of nonlinear equations

Hello there

Can anybody please tell me if there is any package in R to solve the
following 4 nonlinear equations with 4 unknowns:

alpha*exp(20/sigma)+ beta*exp(21/tau) = 2
alpha*exp(22/sigma)+ beta*exp(9/tau) = 4
alpha*exp(10/sigma)+ beta*exp(30/tau) = 6
alpha*exp(40/sigma)+ beta*exp(39/tau) = 5

where 

alpha = exp(lambda/sigma)
beta= exp(delta/tau)

I need to estimate lambda, sigma, delta, tau

Thanks.
E Raheem