//（n(a1 + an)）/ 2
//d = a2 - a1
//an = a1 + (n - 1)*d
#include <bits/stdc++.h>
using namespace std;

int main()
{
    int a1,a2,n;
    cin >> a1 >> a2 >> n;
    int d = a2 - a1;
    int an = a1 + (n - 1) * d;
    cout << an;
    return 0;
}