program abc;
var n,i: integer;
s:real;
begin
readln(n);
s:=0;
for i:=1 to n do
s:=s+1/sqr(2*n+1);
write('s= ', s);
end.
program abc;
var n,i: integer;
s:real;
begin
readln(n);
s:=0;
for i:=1 to n do
s:=s+1/sqr(2*n+1);
write('s= ', s);
end.