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