#include <bits/stdc++.h>
using namespace std;
int c, d;
int n, m;
int k;
int people;
int min(int x, int y) { return x < y ? x : y; };
int main() {
  while (cin >> c >> d) {
    cin >> n >> m;
    cin >> k;
    people = n * m - k;
    if (people <= 0)
      puts("0");
    else if (d * n <= c)
      printf("%d\n", people * d);
    else
      printf("%d\n", people / n * c + min(c, people % n * d));
  }
  return 0;
}
