#include <bits/stdc++.h>
using namespace std;
template <typename Arg1>
void __f(const char* name, Arg1&& arg1) {
  cerr << name << " : " << arg1 << std::endl;
}
template <typename Arg1, typename... Args>
void __f(const char* names, Arg1&& arg1, Args&&... args) {
  const char* comma = strchr(names + 1, ',');
  cerr.write(names, comma - names) << " : " << arg1 << " | ";
  __f(comma + 1, args...);
}
int main() {
  int c, d, n, m, k;
  cin >> c >> d >> n >> m >> k;
  int ans = INT_MAX;
  int want = n * m - k;
  if (want <= 0) {
    printf("%d\n", 0);
    return 0;
  }
  for (int i = 0; i < want + 1; i++) {
    int t = want - i;
    int lim = t / n;
    if (t % n > 0) lim++;
    ans = min(ans, i * d + c * lim);
  }
  printf("%d\n", ans);
  return 0;
}
