#include <bits/stdc++.h>
using namespace std;
int main() {
  int c, d, n, m, k, s;
  while (scanf("%d%d%d%d%d", &c, &d, &n, &m, &k) == 5) {
    s = n * m - k;
    if (s <= 0) {
      printf("0\n");
      continue;
    }
    int ans = 0;
    if (c < d * n) {
      int num = s / n;
      ans += num * c;
      int resman = s % n;
      ans += min(resman * d, c);
    } else {
      ans = s * d;
    }
    printf("%d\n", ans);
  }
  return 0;
}
