Minimization of the distance

I just contect R,and still learn how to write the code.
I have a problem with argmin sum d(pi,p)/n 
for example I have 3 points (a1,b1)(a2,b2)(a3,b3) ,then I want to find
p(x,y) make sure that 
(sqrt((a1-x)^2+(b1-y)^2)+sqrt((a2-x)^2+(b2-y)^2)+sqrt((a3-x)^2+(b3-y)^2))/3
is the minimum.

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