n=gets.chomp.to_i
x=Array.new
y=Array.new
for i in 0..n-1
  x[i],y[i]=gets.chomp.split(" ").map{|s| s.to_i}
end

ans=0.0
for i in 0..n-1
  for j in 0..n-1
    ans+=(((x[i]-x[j])**2)+((y[i]-y[j])**2))**(0.5)
  end
end
p ans/n