#include <stdio.h>

int main()
{

	int N, count = 1, sum = 0, a, b;

	scanf("%d+%d+...", &a, &b);
	scanf("%d", &N);

	int c = b - a;

	for(int i = a; count <= N; i += c) {
		sum += i;
		count++;
	}

	printf("%d", sum);

	return 0;
}