#include <bits/stdc++.h>
int c, d, n, m, k, ans;
int max(int x, int y) { return (x > y) ? x : y; }
int main() {
  int a1, a2, tmp;
  scanf("%d%d%d%d%d", &c, &d, &n, &m, &k);
  ans = 1000000000;
  for (a1 = 0; a1 <= m; a1++) {
    a2 = max(0, n * m - a1 * n - k);
    if ((tmp = a1 * c + a2 * d) < ans) ans = tmp;
  }
  printf("%d\n", ans);
  return 0;
}
