#include <bits/stdc++.h>
using namespace std;
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(nullptr);
  int c, d, n, m, k;
  cin >> c >> d >> n >> m >> k;
  int temp = n * m - k;
  if (temp <= 0) {
    cout << 0;
    return 0;
  }
  if (d < (c + n - 1) / n)
    cout << temp * d;
  else {
    int ans = (temp / n) * c;
    if ((temp % n) * d < c)
      ans += (temp % n) * d;
    else
      ans += c;
    cout << ans;
  }
}
