#include <bits/stdc++.h>

using namespace std;

int main() {
    //cin读入优化
    std::ios::sync_with_stdio(false);
    int a1, a2, n;
    cin >> a1 >> a2 >> n;
    cout << a1 + (a2 - a1) * (n - 1) << endl;
    return 0;
}