#include <bits/stdc++.h>
using namespace std;
int main() {
  int c, d, n, m, k;
  cin >> c >> d >> n >> m >> k;
  if (n * m - k <= 0)
    cout << "0";
  else {
    int cc;
    cc = (((n * m - k) % n == 0) ? ((n * m - k) / n) : ((n * m - k) / n + 1)) *
         c;
    int dd;
    dd = (n * m - k) * d;
    int cd;
    int temp;
    temp = (n * m - k) / n;
    cd = temp * c + ((n * m - k) - temp * n) * d;
    int min = cc;
    if (min > dd) min = dd;
    if (min > cd) min = cd;
    cout << min;
  }
  return 0;
}
