#include <bits/stdc++.h>
using namespace std;
using ll = long long;
signed main() {
  ios::sync_with_stdio(0);
  cin.tie(0);
  int c, d, n, m, k;
  cin >> c >> d >> n >> m >> k;
  int res = 1e9;
  for (int i = 0; i <= m; ++i) {
    int need = max(0, n * m - k - i * n);
    res = min(res, c * i + need * d);
  }
  cout << res << "\n";
  return 0;
}
