#include <bits/stdc++.h>
using std::cin;
int minProblems(int c, int d, int n, int m, int k) {
  int places = m * n - k;
  if (places <= 0) return 0;
  return std::min((places + n - 1) / n * c,
                  std::min(places * d, places / n * c + (places % n) * d));
}
int main() {
  int c, d, m, n, k;
  cin >> c >> d;
  cin >> n >> m;
  cin >> k;
  int result = minProblems(c, d, n, m, k);
  std::cout << result << std::endl;
  return 0;
}
