#include <stdio.h>

int main()
{
	char S[3];
	int N, count = 1, sum = 0;

	scanf("%c+%c+...", &S[0], &S[1]);
	scanf("%d", &N);

	int c = S[1] - S[0], i = S[0] - 48;

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

	}

	printf("%d", sum);

	return 0;
}