#include <bits/stdc++.h>
using namespace std;
int main() {
  int k, c, d, n, m, ans = 0, cnt;
  cin >> c >> d >> n >> m >> k;
  cnt = n * m - k;
  while (cnt > 0) {
    int l = min(cnt, n);
    cnt -= l;
    ans += min(c, l * d);
  }
  cout << ans << endl;
  return 0;
}
