#include<bits/stdc++.h>
using namespace std;
using ll = long long;
using dt = array<int, 2>;

ll sum(ll a, ll d, ll n) {
  return n * (2 * a + (n - 1) * d) / 2;
}

void test() {
  string s;
  cin >> s;

  // cout << s << "\n";

  vector<ll> num;
  string v;
  for (auto e : s) {
    if (e >= '0' and e <= '9') {
      v += e;
    } else {
      if (v.size()) {
        // cout << v << "\n";
        num.push_back(stoll(v));
        v.clear();
        if (num.size() == 2) break;
      }
    }
  }

  ll m;
  cin >> m;

  // cout << num.front() << " " << num.back() << "\n";

  cout << sum(num[0], num[1] - num[0], m) << "\n";
}

int main() {
  ios::sync_with_stdio(false);
  cin.tie(nullptr);
  int T = 1;  
  // cin >> T;
  for (int i = 1; i <= T; i++) {
    test();
  }
  return 0;
}