#include <bits/stdc++.h>
using namespace std;
typedef long long ll;

signed main() {
    ios_base::sync_with_stdio(0); cin.tie(0);
   
    string s; cin >> s;
    ll n; cin >> n;

    vector<ll> v;
    string temp = "";
    for (auto &u : s) {
        if (isdigit(u)) temp += u;
        else {
            if (!temp.empty()) {
                v.push_back(stoll(temp));
                temp.clear();
            }
        }
    }
    if (!temp.empty()) v.push_back(stoll(temp));

    ll a1 = v[0];
    ll d = v[1]-v[0];

    cout << (n * (2*a1 + (n-1) * d)) / 2;
}