#include <bits/stdc++.h>
using namespace std;
int main() {
  int c, d, n, m, k, needed;
  cin >> c >> d >> n >> m >> k;
  needed = n * m - k;
  cout << ((needed <= 0)
               ? 0
               : min(needed * d, min(needed / n * c + needed % n * d,
                                     (int)(ceil((double)needed / n) * c))));
  return 0;
}
