#include <bits/stdc++.h>
#pragma comment(linker, "/stack:102400000,102400000")
using namespace std;
const int N = 100005;
const int M = 200005;
const int INF = 0x3f3f3f3f;
const int MOD = 1000000007;
const double Pi = acos(-1.0);
int lowbit(int x) { return x & (-x); }
template <class T>
inline T f_min(T a, T b) {
  return a < b ? a : b;
}
template <class T>
inline T f_max(T a, T b) {
  return a > b ? a : b;
}
template <class T>
inline T f_abs(T a) {
  return a > 0 ? a : -a;
}
template <class T>
inline T f_gcd(T a, T b) {
  return b == 0 ? a : f_gcd(b, a % b);
}
template <class T>
inline T f_Lcm(T a, T b) {
  return a / f_gcd(a, b) * b;
}
template <class T>
inline void f_swap(T &a, T &b) {
  T t = a;
  a = b;
  b = t;
}
bool is_digit(char a) {
  if (a >= '0' && a <= '9') return true;
  return false;
}
bool is_lowch(char a) {
  if (a >= 'a' && a <= 'z') return true;
  return false;
}
bool is_upch(char a) {
  if (a >= 'A' && a <= 'Z') return true;
  return false;
}
int main() {
  int c, d;
  while (cin >> c >> d) {
    int n, m, k;
    cin >> n >> m >> k;
    int sum = n * m - k, res = INF;
    for (int i = 0; i <= sum / n + 2; ++i) {
      int ans = c * i;
      if (sum - i * n > 0) ans += (sum - i * n) * d;
      res = f_min(res, ans);
    }
    cout << (res == INF ? 0 : res) << endl;
  }
  return 0;
}
