#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;

    int pos = s.find('+');
    long long x = stoll(s.substr(0, pos));
    int pos2 = s.find('+', pos + 1);
    long long y = stoll(s.substr(pos + 1, pos2 - pos - 1));

    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;
}