#include <bits/stdc++.h>
const bool debugging = 1;
using namespace std;
template <class T>
ostream &operator<<(ostream &os, vector<T> V) {
  for (auto v : V) {
    os << v << " ";
  }
  return os << "";
}
template <class T>
ostream &operator<<(ostream &os, set<T> S) {
  for (auto s : S) {
    os << s << " ";
  }
  return os << "";
}
template <class L, class R>
ostream &operator<<(ostream &os, pair<L, R> P) {
  return os << P.first << " " << P.second;
}
template <class L, class R>
ostream &operator<<(ostream &os, map<L, R> M) {
  os << "\n";
  for (auto m : M) {
    os << m << "\n";
  }
  return os << "";
}
int32_t main() {
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
  long long c, d, n, m, k;
  cin >> c >> d >> n >> m >> k;
  long long need = n * m - k;
  if (need <= 0) {
    cout << 0 << "\n";
    return 0;
  }
  long long ans = INT64_MAX;
  for (long long i = 0; i <= m; i++) {
    long long temp = need - n * i;
    if (temp <= 0) {
      ans = min(ans, i * c);
      continue;
    }
    ans = min(ans, i * c + temp * d);
  }
  cout << ans << "\n";
  return 0;
}
