#include <bits/stdc++.h>
using namespace std;
int main() {
  long long ans = 0, n, m, k, r, f;
  double c, d;
  cin >> c >> d;
  cin >> n >> m;
  cin >> k;
  if (k >= n * m) {
    cout << "0\n";
    return 0;
  }
  r = n * m - k;
  if (c / n >= d) {
    cout << r * d << endl;
    return 0;
  }
  if (r % n == 0) {
    cout << c * (r / n) << endl;
    return 0;
  }
  f = floor(r / n);
  ans += f * c;
  ans += min(c, (r - f * n) * d);
  cout << ans << endl;
  return 0;
}
