#include<bits/stdc++.h>
using namespace std;
int main()
{
	long long ans,tc=0,sum=0;
	long long n,frt,scd;
	long long i,j;
	cin>>frt;
	cin>>scd;
	cin>>n;
	tc=scd-frt;
	sum=frt;
	ans=frt;
	for(i=2;i<=n;i++)
	{
		sum=sum+frt;
		ans+=sum;
	}
	cout<<ans;
}