#include <bits/stdc++.h>
using namespace std;
int main() {
  int c, d, n, m, k, sum = 0, z = 0;
  while (cin >> c >> d >> n >> m >> k) {
    sum = n * m;
    if (sum <= k)
      cout << "0" << endl;
    else {
      sum = sum - k;
      while (sum > 0) {
        if (sum >= n) {
          if (c / n <= d) {
            sum = sum - n;
            z = z + c;
          } else {
            sum = sum - 1;
            z = z + d;
          }
        } else {
          if (sum * d >= c) {
            sum = sum - n;
            z = z + c;
          } else {
            sum = sum - 1;
            z = z + d;
          }
        }
      }
      cout << z << endl;
    }
    z = 0;
    sum = 0;
  }
}
