#include <bits/stdc++.h>
using namespace std;
int main() {
  int n, ans = 0, c, d, m, k, min = INT_MAX, l, r;
  scanf("%d%d%d%d%d", &c, &d, &n, &m, &k);
  for (int i = 0; i <= 10000; i++)
    for (int j = 0; j <= 10000; j++)
      if ((k + i * n + j >= n * m) && ((i * c + j * d) < min))
        min = i * c + j * d;
  printf("%d", min);
  return 0;
}
