#include <bits/stdc++.h>
using namespace std;
long long gcd(long long x, long long y) { return !y ? x : gcd(y, x % y); }
int main() {
  int c, d, n, m, k, tmp, ans;
  cin >> c >> d >> n >> m >> k, tmp = n * m - k;
  if (tmp < 0) {
    cout << 0 << endl;
    return 0;
  }
  ans = min((tmp / n) * c, tmp * d);
  if ((tmp / n) * c < tmp * d)
    tmp -= (ans / c) * n;
  else
    tmp -= ans * d;
  if (tmp > 0) ans += min((int)ceil((float)tmp / n) * c, tmp * d);
  cout << ans << endl;
  return 0;
}
