#include <bits/stdc++.h>
using namespace std;
using vi = vector<int>;
using vvi = vector<vi>;
using ii = pair<int, int>;
using vii = vector<ii>;
using l = long long;
using vl = vector<l>;
using vvl = vector<vl>;
using lu = unsigned long long;
using vb = vector<bool>;
using vvb = vector<vb>;
using vd = vector<double>;
using vvd = vector<vd>;
const int INF = numeric_limits<int>::max();
const double EPS = 1e-10;
const l e5 = 100000, e6 = 1000000, e7 = 10000000, e9 = 1000000000;
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(0);
  l c, d, n, m, k;
  while (cin >> c >> d >> n >> m >> k) {
    l p = INF;
    for (l i = 0; i <= m; i++) {
      l j = n * m - i * n - k;
      j = max(j, l(0));
      p = min(p, i * c + j * d);
    }
    cout << p << endl;
  }
}
