#include <bits/stdc++.h>
using namespace std;
using ll = long long;
using pii = pair<int, int>;
using vi = vector<int>;
using ld = long double;
using pll = pair<ll, ll>;
using vll = vector<ll>;
using pll = pair<ll, ll>;
void buff() {
  ios::sync_with_stdio(false);
  cin.tie(nullptr);
}
constexpr int MOD = 1e9 + 7;
inline ll pow_mod(ll a, ll b, ll mod = MOD) {
  ll res = 1;
  a %= mod;
  assert(b >= 0);
  for (; b; b >>= 1) {
    if (b & 1) res = (res * a) % mod;
    a = (a * a) % mod;
  }
  return res;
}
int main() {
  buff();
  int c, d;
  cin >> c >> d;
  int n, m;
  cin >> n >> m;
  int k;
  cin >> k;
  int tot = n * m - k;
  if (tot <= 0) {
    cout << 0 << '\n';
    return 0;
  }
  int res = MOD;
  for (int i = 0; i <= n * m; ++i) {
    for (int j = 0; j <= n * m; ++j) {
      int tot_pass = i * n + j;
      if (tot_pass < tot) continue;
      res = min(res, i * c + j * d);
    }
  }
  cout << res << '\n';
  return 0;
}
