\begin{array}{c} \color{#FFFFFFff}{H}\color{#E7E7E7ff}{o}\color{#CFCFCFff}{r}\color{#B7B7B7ff}{i}\color{#9F9F9Fff}{z}\color{#878787ff}{o}\color{#6F6F6Fff}{n} \end{array} $\\$

// Horizon
#include <bits/stdc++.h>
using namespace std;

int main() {
    long long a, b, k; cin >> a >> b >> k;

    if (k % 2 == 0) cout << (k / 2) * (a - b);
    else cout <<  (k / 2) * (a - b) + a;
}