var
i, n: integer;
x, s, a, xn: real;
begin
read(n, x);
a := 1;
s := 1;
xn := 1;
for i := 1 to n do
a := a * (2 * i - 1) / sqr (2 * i);
xn := xn * x;
s := s - a * xn;
end;
write(s * Pi / 2);
end.
var
i, n: integer;
x, s, a, xn: real;
begin
read(n, x);
a := 1;
s := 1;
xn := 1;
for i := 1 to n do
begin
a := a * (2 * i - 1) / sqr (2 * i);
xn := xn * x;
s := s - a * xn;
end;
write(s * Pi / 2);
end.