function [f,nlnl]=WALD_PDF(x,par)
if min(x)<=0
    f=[];
    return
end
mu=par(1);
lambda=par(2);
nlnl=-log(sqrt(lambda/2/pi)) +3/2*log(x) +lambda*(x-mu).^2/2/mu^2./x;
f=exp(-nlnl);