#include <bits/stdc++.h>
using namespace std;
int main() {
  int c, d, n, m, k;
  cin >> c >> d >> n >> m >> k;
  int all = n * m - k;
  if (all < 0) {
    cout << 0;
    return 0;
  }
  if (c < d * n || (c < d && all < n)) {
    int rounds = all / n;
    if (rounds * n == all) {
      cout << rounds * c;
      return 0;
    }
    all -= rounds * n;
    if (all * d < c) {
      cout << rounds * c + all * d;
    } else {
      cout << (1 + rounds) * c;
    }
  } else {
    cout << all * d;
  }
  return 0;
}
