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