#include <bits/stdc++.h>
using namespace std;
const int maxn = 114514;
int a1, a2, n, sn, d, an;
int main() {
	cin >> a1 >> a2 >> n;
	d = a2 - a1;
	an = a1 + (n - 1) * d;
	sn = (a1 + an) * n * 1.0 / 2;
	cout << sn;
}