#include <bits/stdc++.h>
using namespace std;
#define endl '\n'
int main()
{
    ios_base::sync_with_stdio(0);
    cin.tie(0);

    string s;
    cin >> s;

    long long x = s[0] - '0';
    long long y = s[2] - '0';

    long long dif = y - x;
    long long n;
    cin >> n;

    long long part1 = (n - 1) * x + x;
    long long sum = (n * (n - 1) / 2) * dif;
    sum += part1;

    cout << sum << endl;
    return 0;
}