#include <bits/stdc++.h>
using namespace std;
int main() {
  int c, d, n, m, k;
  cin >> c >> d >> n >> m >> k;
  int need = n * m - k;
  int tasks = 0;
  if (need > 0) {
    if (d * n < c) {
      while (need > 0) {
        tasks += d;
        need--;
      }
    } else {
      while (need / n > 0) {
        tasks += c;
        need -= n;
      }
      if (need > 0) {
        tasks += min(c, need * d);
      }
    }
  }
  cout << tasks << endl;
  return 0;
}
