#include <Stdio.h>

int main()
{
	int a1, a2, n, ans = 0, d;
	scanf("%d%d%d", &a1, &a2, &n);
	d = a2 - a1;
	for (int i = 1, temp = a1; i <= n; i++)
	{
		ans = temp + ans;
		temp = d + temp;
	}
	printf("%d\n", ans);
	return 0;
}