#include <bits/stdc++.h>
using namespace std;
int main() {
  int i, j, k, c, d, n, m, num, sum;
  scanf("%d%d%d%d%d", &c, &d, &n, &m, &k);
  num = n * m - k;
  sum = 0;
  if (num <= 0)
    sum = 0;
  else {
    while (num != 0) {
      if (num >= n) {
        if (c < n * d)
          sum += c;
        else
          sum += n * d;
        num = num - n;
      } else if (num < n) {
        if (c < num * d)
          sum += c;
        else
          sum += num * d;
        num = 0;
      }
    }
  }
  printf("%d\n", sum);
}
