#include <bits/stdc++.h>
using namespace std;
int main() {
  int c, d, n, m, k;
  cin >> c >> d >> n >> m >> k;
  int ans = 1e9;
  for (int x = 0; x <= m; ++x) {
    int y = n * m - k - n * x;
    if (y < 0) y = 0;
    ans = min(ans, c * x + d * y);
  }
  cout << ans << endl;
  return 0;
}
