I have the model:

I1 <- inla(n ~ median + weight + length,
           control.predictor = list(compute = TRUE),  
           quantiles = c(0.025, 0.975),
           control.compute=list(dic = TRUE, waic = TRUE, cpo=TRUE, return.marginals.predictor=TRUE),
           family = "poisson", 
           data = df)

and I want to specify priors only for length (I believe that it should have negative coefficient).
How can I do it?