#include <bits/stdc++.h>
using namespace std;
int dp[100005];
int main() {
  int c, d, n, m, k;
  scanf("%d%d", &c, &d);
  scanf("%d%d", &n, &m);
  scanf("%d", &k);
  int rem = n * m - k;
  if (rem <= 0) rem = 0;
  int qus = c * (rem / n) < d * rem ? c * (rem / n) : d * rem;
  if (qus == d * rem)
    rem = 0;
  else
    rem %= n;
  qus = qus + c < qus + (d * rem) ? qus + c : qus + (d * rem);
  printf("%d\n", qus);
  return 0;
}
