#include <iostream>
#include <cmath>
using namespace std;
double func(double n)
{
return cos(n) + n;
}
int main()
{
int a,b,m; cin >> a >> b >> m;
double sum = 0.0;
double h = (b - a) / (double)m;
for(int i = 1; i < m; i++)
{
sum += func(a + i * h);
}
cout << ((func(a) + func(b)) / 2 + sum) * h;
return 0;
}