#include <bits/stdc++.h>
using namespace std;
const int INF = 0x3f3f3f3f;
const double eps = 1e-10;
const int MOD = 100000007;
const int MAXN = 1000010;
const double PI = acos(-1.0);
const int MAXV = 1100;
int main() {
  int c, d, n, m, k;
  scanf("%d%d%d%d%d", &c, &d, &n, &m, &k);
  int all = n * m - k;
  if (all <= 0) {
    puts("0");
  } else if (c < n * d) {
    int ans = all / n * c;
    all -= all / n * n;
    if (all != 0) {
      if (c < all * d)
        ans += c;
      else
        ans += all * d;
    }
    cout << ans << endl;
  } else {
    cout << d * all << endl;
  }
  return 0;
}
