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