Syntax for total derivatives

I have this equation -

(D1*p*(d^2 f1/dC1^2) + D2*(1-p)*(d^2 f2/dC2^2))

d^2 / dC^2 represents a second order total derivative.
f1 and f2 are functions where
f1 = f(C1, Ca)
f2 = f(C2, Cb)
In general, How do I type the syntax for total derivatives?
It would be of great help if you could help me type the syntax for the above equation

Please help. Thanks