#include <iostream>
#include<cmath>
#include<fstream>


using namespace std;



int main()
{
    int first, second, neednumb,d;

    ifstream fin("INPUT.txt");
    ofstream fout("OUTPUT.txt");
    fin >> first >> second >> neednumb;

    d = second - first;

    if (first<0||first>1000||second<0||second>1000)
    {
        return 0;
    }

    fout << first + d * (neednumb - 1);








}