#include <bits/stdc++.h>
using namespace std;
int c, d, n, m, k, req, cnt = 0, minn = 1e9;
int main() {
  scanf("%d%d%d%d%d", &c, &d, &n, &m, &k);
  req = n * m - k;
  for (int i = 0;; i++) {
    int j = max(req - i * n, 0);
    if (j + i * n >= req) minn = min(minn, j * d + i * c);
    if (i * n >= req) break;
  }
  printf("%d", minn);
}
