#include <bits/stdc++.h>
int main() {
  int c, d, n, m, k, ans, temp;
  while (~scanf("%d", &c)) {
    ans = 0;
    scanf("%d%d%d%d", &d, &n, &m, &k);
    k = n * m - k;
    if (k <= 0) {
      printf("0\n");
    } else if (d * n <= c) {
      ans = k * d;
      printf("%d\n", ans);
    } else {
      temp = k / n;
      ans = c * temp;
      k -= n * temp;
      if (k > 0 && k * d <= c)
        ans += k * d;
      else if (k > 0)
        ans += c;
      printf("%d\n", ans);
    }
  }
  return 0;
}
