#include <bits/stdc++.h>
using namespace std;
const int MAX_N = 100000 + 10;
int c, d, n, m, k;
int main() {
  ios_base::sync_with_stdio(false);
  cin >> c >> d >> n >> m >> k;
  int p = n * m;
  p -= k;
  int MIN = 9999999;
  if (p <= 0) {
    cout << "0";
    return 0;
  }
  for (int i = 0; i <= p; i++) {
    int ans = 0;
    ans += i * d;
    int q = p - i;
    int s = 0;
    s = q / n;
    if (q % n != 0) s++;
    ans += s * c;
    MIN = min(MIN, ans);
  }
  cout << MIN;
}
