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