#include <bits/stdc++.h>
using namespace std;
int main() {
  int c, d;
  cin >> c >> d;
  int n, m;
  cin >> n >> m;
  int k;
  cin >> k;
  int p = n * m;
  if (p <= k) {
    cout << 0;
  } else if (c / n >= d) {
    cout << (p - k) * d;
  } else {
    int r = (p - k) % n;
    if (r != 0) {
      if (c / r >= d) {
        cout << ((int)(p - k) / n) * c + r * d;
      } else {
        cout << (((int)(p - k) / n) + 1) * c;
      }
    } else
      cout << (p - k) / n * c;
  }
}
