#include <bits/stdc++.h>
using namespace std;
int c, d, n, m, k, rez;
int main() {
  int x, y;
  scanf("%d %d", &c, &d);
  scanf("%d %d", &n, &m);
  scanf("%d", &k);
  x = (n * m - k) / n + 1;
  if (x < 0) x = 0;
  y = 0;
  rez = x * c;
  for (x; x >= 0; --x) {
    y = n * m - k - x * n;
    if (y < 0) y = 0;
    if (x * n + y < n * m - k) break;
    if (x * c + y * d < rez) rez = x * c + y * d;
  }
  printf("%d", rez);
  return 0;
}
