#include <bits/stdc++.h>
using namespace std;
using ll = long long;
int main () {
    // A(N-4b-1)-B(N-2b) = (2B - 4A)b 
    ll A, B, N;
    cin >> A >> B >> N;
    ll ans = 0;
    if (N % 2 == 0) {
        ans = A * (1 - N);
    }
    if (2*B - 4*A < 0) {
        ll b = (N-1)/2;
        ans = min(ans, A * (N - 4*b - 1) - B * (N - 2*b));
    } else {
        ll b = 0;
        ans = min(ans, A * (N - 4*b - 1) - B * (N - 2*b));
    }
    cout << ans << endl;
}