I got a cdf in below form:

--- 1 - exp(-a*x) - exp(-(a+b)*y) + exp(-(a*x + b*y)), x>=y>0
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F(x,y) = --- 1 - exp(-a*x), 0<x<y
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--- 0, others

It seems this cdf is not continuous, and I can't simply get the pdf by taking the second order derivative.

Can anyone tell me whether it is possible to get pdf from it? And if possible, how?

Thank you very much in advance. Any comments are welcomed.