#include <bits/stdc++.h>
using namespace std;
int c, d, n, m, k;
int main() {
  scanf("%d %d %d %d %d", &c, &d, &n, &m, &k);
  int sum;
  sum = n * m - k;
  if (sum <= 0) {
    printf("0\n");
    return 0;
  }
  int ans;
  int t = sum / n;
  int aa = sum % n;
  ans = t * min(c, n * d) + min(aa * d, c);
  printf("%d\n", ans);
  return 0;
}
