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