#include <bits/stdc++.h>
using namespace std;
int32_t main() {
  ios_base::sync_with_stdio(0);
  cin.tie(0);
  cout.tie(0);
  long long c, d, n, m, k;
  cin >> c >> d >> n >> m >> k;
  long long need = n * m - k;
  if (need <= 0) {
    cout << "0" << endl;
    return 0;
  }
  long long l = ceil(need / float(n));
  long long ans = c * l;
  ans = min(ans, need * d);
  long long rem = need % n;
  ans = min(ans, (d * rem) + c * (need / n));
  cout << ans;
  return 0;
}
