What is the MLE of $~(x_1,\cdots,x_n)~$ realizations with pdf $~λ~x^{-2}~$?

Question

I am trying to find the maximum likelihood estimator for parameter $~λ~$ in the above, I am pretty sure differentiation will not work in this case, so there should be another way. Any ideas?

my pfd is $~λ~x^{-2}~$, so the joint pdf I found it to be: $~λ^n ~ Π(xi)^{-2}~$