#include <bits/stdc++.h>
using namespace std;
const long long maxn = 4e6;
const long double SADAT = 7.84e-17;
const long long INF = 1e9;
const long long mod = 1e9 + 7;
const long double PI = 4 * atan((long double)1);
long long pw(long long a, long long b) {
  return (!b ? 1
             : (b & 1 ? a * pw(a * a % mod, b / 2) % mod
                      : pw(a * a % mod, b / 2) % mod));
}
long long c, d, n, m, k;
int main() {
  ios::sync_with_stdio(false);
  cin.tie(0);
  cout.tie(0);
  cin >> c >> d >> n >> m >> k;
  long long w = n * m - k;
  long long mn = 1e9;
  for (long long i = 0; i < 10005; i++) {
    mn = min(mn, c * i + max(w - n * i, (long long)0) * d);
  }
  cout << mn;
  return (0);
}
