#include <bits/stdc++.h>
int main() {
  int c = 0, d = 0, n = 0, m = 0, k = 0;
  std::cin >> c >> d >> n >> m >> k;
  int p = 0;
  int a = n * m - k;
  if (a > 0) {
    int p1 = (a + n - 1) / n * c;
    int p2 = a * d;
    int p12 = a / n * c + a % n * d;
    p = std::min(std::min(p1, p2), p12);
  }
  std::cout << p;
  return 0;
}
