program ideone;
type TFunc = function (x:Real):Real;

function trapezoid(f:TFunc;a,b:Real;n:longint):Real;
var k:longint;
	s,h:Real;
begin
  s := f(a) + f(b);
  h := (b-a)/n;
  for k := 1 to n - 1 do 
     s := s + 2 * f(a + k * h);
  trapezoid := 0.5 * s * h;
end;

function f1(x:Real):Real;
begin
  f1 := ln(1+x)/(1+sqr(x));
end;

function f2(x:Real):Real;
begin
  f2 := ln(sin(x))*ln(cos(x))/sin(2*x);
end;

var a,b:Real;
	n:longint;

begin
	(* your code goes here *)
	readln(a,b);
	readln(n);
	writeln(trapezoid(f2,a,b,n):1:10);
end.