local function reada(n,m)m=m or 1 r={}for i=1,m do r[i]={}end for i=1,n do for j=1,m do r[j][i]=io.read"*n"end end return unpack(r)end
N=io.read"*n"
x,y=reada(N,2)
function d(x1,y1,x2,y2)return math.sqrt((x2-x1)*(x2-x1)+(y2-y1)*(y2-y1))end

s=0
dt={}
for i=1,N do
dt[i]={}
for j=1,i-1 do
dt[i][j]=d(x[i],y[i],x[j],y[j])
s=s+dt[i][j]
dt[j][i]=dt[i][j]
end
end
print(s/N*2)