#include <bits/stdc++.h>
using namespace std;
int main() {
  int c, d, n, m, k;
  scanf("%d%d%d%d%d", &c, &d, &n, &m, &k);
  int sum = n * m - k, ans = 0;
  if (sum <= 0)
    ans = 0;
  else if (c / (1.0 * n) >= d)
    ans = sum * d;
  else {
    ans += (sum / n * c);
    ans += min(c, (sum % n) * d);
  }
  printf("%d", ans);
  return 0;
}
