import java.util.*;
import java.lang.*;
import java.io.*;
 
class Ideone
{
	public static double f(double x){return (-3*x*x+2*x+9);}
	public static double I(double a,double b,int n,double y){return ((b-a)/(2*n)*y);}
	public static void main (String[] args) throws java.lang.Exception
	{
		Scanner in = new Scanner(System.in);
		int n; double a,b,dy,In,y=0;
		a = in.nextDouble();
		b = in.nextDouble();
		n = in.nextInt();
		if (n>1){
			dy=(b-a)/n;
			y+=f(a)+f(b);
			for (int i=1; i<n; i++) {y+=2*(f(a+dy*i));}
			In=I(a,b,n,y);
			System.out.printf("%f", In);
		}
		else {System.out.printf("Wrong Data");}
	}
}

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LTEgMiAyNQ==

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