#include <iostream>
#include <math.h>
using namespace std;
double f(double x) { return (-3 * x*x + 2 * x + 9); }
double I(double a, double b, int n, double y) { return ((b - a) / (2 * n)*y); }
int main() {
	int n; double a, b, y, dy, In;
	cin >> a >> b >> n;
	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);
		cout << In;
	}
	else { cout << "Wrong data"; }
}