#include <bits/stdc++.h>

using namespace std;
//给出等差数列的前两项，分别是a1,a2,问第n项是多少
int x, y, n;

int main() {
    cin >> x >> y >> n;
    int z = y - x;
    if (n == 1) cout << x << endl;
    if (n == 2) cout << y << endl;
    if (n >= 3) {
        for (int i = 3; i <= n; i++)y += z;
        cout << x << endl;
    }
    return 0;
}