#include <bits/stdc++.h>
using namespace std;
const int N = 111;
const int INF = 0x3f3f3f3f;
const int MOD = 1e9 + 7;
int main() {
  int c, d, n, m, k;
  cin >> c >> d >> n >> m >> k;
  int s = n * m - k;
  int ans;
  if (s <= 0) {
    ans = 0;
  } else {
    ans = (s / n) * min(c, d * n) + min(c, (s % n) * d);
  }
  cout << ans << endl;
  return 0;
}
