#include <bits/stdc++.h>
using namespace std;
int main() {
  int c, d, n, m, k, ans, i, j;
  cin >> c >> d;
  cin >> n >> m;
  cin >> k;
  ans = n * m * d;
  if (k >= n * m) {
    cout << "0";
    return 0;
  }
  for (i = k; i <= n * (m + 1); i += n) {
    ans = min(ans, ((i - k) / n) * c + max((n * m - i) * d, 0));
  }
  cout << ans;
  return 0;
}
