uses crt;
var a, b:integer; result: real;
function x_fact(n:integer):longint;
var r:longint; i:integer;
begin
r := 1;
for i := 1 to n do if (i mod 2 = n mod 2) then r := r * i;
x_fact := r;
end;
begin
clrscr;
write('A, B -> ');
readln(a, b);
result := 1 - 2 * b / (x_fact(a-2) + b);
writeln('RESULT = ',result:0:5);
readln;
end.