program xxxx;
type vector=array[1..100] of integer;
var  a,b:vector;
z,x,y,s:real;
i,j,n,k:longint;
begin
readln(n);
readln(k);
s:=0;
z:=0;
for i:=1 to n do
begin
read(a[i]);
write(' ');
end;
writeln;
for j:=1 to n do
for i:=1 to n do
b[j]:=a[i];
for i:=1 to n do
for j:=1 to n do
if i<>j then
begin
z:=b[j]*a[i];
x:=x+1;
s:=s+z;
end;
write((s div x):4:6);
readln;
end.