// @rakibul-islam

#include "bits/stdc++.h"
using namespace std;

using ll = long long;
const ll oo = 1e17;

// 1 + 3 + 5 + 7 + 9

int main() {
  cin.tie(0)->sync_with_stdio(0);
  string s; cin >> s;
  int n;  cin >> n;
  ll fst = 0, snd = 0;
  int i = 0;
  while (s[i] != '+') {
    fst *= 10;
    fst += int(s[i] - '0');
    i++;
  }
  i++;
  while (s[i] != '+') {
    snd *= 10;
    snd += int(s[i] - '0');
    i++;
  }
  ll dif = (snd - fst);
  ll add = dif * (n - 1);
  ll N = fst + add;
  ll sum = n * (fst + N);
  sum /= 2;
  cout << sum << "\n";
  return 0;
}