#include <bits/stdc++.h>
using namespace std;
template <class T>
inline void si(T &x) {
  register int c = getchar();
  x = 0;
  int neg = 0;
  for (; ((c<48 | c> 57) && c != '-'); c = getchar())
    ;
  if (c == '-') {
    neg = 1;
    c = getchar();
  }
  for (; c > 47 && c < 58; c = getchar()) {
    x = (x << 1) + (x << 3) + c - 48;
  }
  if (neg) x = -x;
}
long long bigmod(long long p, long long e, long long M) {
  long long ret = 1;
  for (; e > 0; e >>= 1) {
    if (e & 1) ret = (ret * p) % M;
    p = (p * p) % M;
  }
  return ret;
}
long long gcd(long long a, long long b) {
  if (b == 0) return a;
  return gcd(b, a % b);
}
long long modinverse(long long a, long long M) { return bigmod(a, M - 2, M); }
void io() { freopen("/Users/MyMac/Desktop/in.txt", "r", stdin); }
const int N = 1e4 + 5;
int c, d, n, m;
int k;
int need;
int dp[N];
int calc(int po) {
  if (po >= need) return 0;
  if (dp[po] != -1) return dp[po];
  int r = ((c + calc(po + n)) > (d + calc(po + 1)) ? (d + calc(po + 1))
                                                   : (c + calc(po + n)));
  return dp[po] = r;
}
int main() {
  si(c);
  si(d);
  si(n);
  si(m);
  si(k);
  need = n * m - k;
  memset(dp, -1, sizeof(dp));
  cout << calc(0);
  return 0;
}
