#include <bits/stdc++.h>
using namespace std;
const int N = 1000010;
int gcd(int a, int b) { return b ? gcd(b, a % b) : a; }
int lcm(int a, int b) { return a * b / gcd(a, b); }
int main(void) {
  int c, d, n, m, k, i, j, ans = 10000;
  scanf("%d %d %d %d %d", &c, &d, &n, &m, &k);
  for (i = 0; i <= 10000; i++)
    for (j = 0; j <= 10000; j++)
      if (i * n + j >= n * m - k) ans = min(ans, i * c + j * d);
  cout << ans << endl;
  return 0;
}
