#include <bits/stdc++.h>
using namespace std;
int main() {
  int i, j, k, l, t, r, s, m, n, a, b, c, d, e, f, g, h, u, v;
  scanf("%d %d", &c, &d);
  scanf("%d %d", &n, &m);
  scanf("%d", &k);
  a = n * m;
  a = a - k;
  if (a > 0) {
    e = a / n;
    h = 100000000;
    g = 0;
    if (a % n > 0) ++e;
    for (i = 0; i <= e; i++) {
      g = i * c;
      if (a - (i * n) >= 0) {
        g = g + (a - (i * n)) * d;
      }
      if (g < h) h = g;
    }
    printf("%d\n", h);
  } else
    printf("0\n");
  return 0;
}
