#include <bits/stdc++.h>
#define MOD 998244353
#define mod 1000000007
#define ll long long
#define ull unsigned long long
#define ui unsigned int
#define range(i,a,b,C) for (int i = a; i <= b; i += C)
#define round(i,a,b,C) for (int i = a; i < b; i += C)
using namespace std;
int a,b,sum = 0,C,i;
int main()
{
	cin >> a >> b >> C;
	sum = a;
	range(i,2,C,1) sum += (b - a);
    cout << sum << endl;
	return 0;
}