#include <bits/stdc++.h>
using namespace std;
int main() {
  int c, d;
  int n, m, k;
  int num, res;
  while (scanf("%d %d", &c, &d) == 2) {
    scanf("%d %d", &n, &m);
    scanf("%d", &k);
    num = n * m - k;
    res = 0;
    if (num > 0) {
      while (num >= n) {
        if (c / n < d) {
          res += c;
          num -= n;
        } else {
          res += num * d;
          num = 0;
          break;
        }
      }
      if (num > 0) {
        num* d < c ? res += num* d : res += c;
      }
    }
    cout << res << endl;
  }
  return 0;
}
