#include <bits/stdc++.h>
using namespace std;
int n, m, k, c, d;
void solve() {
  int ans = 0;
  int sum = n * m - k;
  double t1 = (double)c / n;
  double t2 = d;
  if (t2 > t1 && sum >= 0) {
    int n1 = sum / n;
    sum -= (n1 * n);
    ans += n1 * c;
    ans += min(c, sum * d);
  } else if (sum >= 0)
    ans = sum * d;
  printf("%d\n", ans);
}
int main() {
  scanf("%d %d %d %d %d", &c, &d, &n, &m, &k);
  solve();
  return 0;
}
