# include <stdio.h>
# include <stdlib.h>
int main()

{

	long long a1,a2,n,sum,d;
	scanf("%l64d%l64d%l64d",&a1,&a2,&n);
	int num[10000];
	d=a2-a1;
	num[0]=a1;num[1]=a2;
	sum=a1+a2;
	for(int i=2;i<n;i++)
	{
		num[i]=a2+d;
		sum+=num[i];
	}
	printf("%ld",sum);
	return 0;
}