#include <bits/stdc++.h>
using namespace std;
int c, d;
int n, m;
int k;
int main() {
  int i, j;
  int ans = 1000000000;
  cin >> c >> d;
  cin >> n >> m;
  cin >> k;
  for (i = 0; i <= m; ++i) {
    j = max(0, n * (m - i) - k);
    ans = min(ans, i * c + j * d);
  }
  cout << ans << endl;
  return 0;
}
