#include <bits/stdc++.h>
using namespace std;
int main() {
  double c, d;
  int n, m, k;
  scanf("%lf %lf", &c, &d);
  scanf("%d %d %d", &n, &m, &k);
  int left = n * m - k;
  int cnt = 0;
  if (left <= 0) {
    printf("0");
    return 0;
  }
  if (n / c > 1 / d) {
    cnt = (left / n) * c;
    left %= n;
    if (c > left * d)
      cnt += left * d;
    else
      cnt += c;
  } else {
    cnt += left * d;
  }
  printf("%d\n", cnt);
}
