#include <bits/stdc++.h>
using namespace std;
int main() {
  int c, d, n, m, k;
  cin >> c >> d >> n >> m >> k;
  if (d * n < c) {
    cout << max((n * m - k) * d, 0);
  } else {
    int ost = n * m - k;
    if (ost <= 0) {
      cout << 0;
    } else {
      int kol = ost / n;
      int kkol = ost - kol * n;
      int ans = kol * c;
      ans += min(c, kkol * d);
      cout << ans;
    }
  }
  return 0;
}
