#include <bits/stdc++.h>
using namespace std;
const int INF = 0x3f3f3f3f;
const double PI = 3.14159265358979323846;
int main() {
  int c, d, n, m, k;
  scanf("%d%d%d%d%d", &c, &d, &n, &m, &k);
  int cnt = n * m;
  if (cnt <= k)
    printf("0");
  else {
    cnt -= k;
    if (cnt == 1)
      printf("%d", min(c, d));
    else {
      int r = cnt / n;
      if (cnt - n * r == 0)
        printf("%d", min(c * r, cnt * d));
      else {
        int t = cnt - n * r;
        printf("%d", min(min(c * r + t * d, c * (r + 1)), cnt * d));
      }
    }
  }
  return 0;
}
