#include <bits/stdc++.h>
using namespace std;
const int N = 30;
long long power_mod(long long x, long long y) {
  x %= 1000000007;
  long long res = 1;
  while (y) {
    if (y & 1) {
      res = (res * x) % 1000000007;
    }
    y /= 2;
    x = (x * x) % 1000000007;
  }
  return res;
}
void solve() {
  int ans = INT_MAX;
  int c, d, n, m, k;
  cin >> c >> d >> n >> m >> k;
  int total = n * m - k;
  if (total <= 0) {
    cout << "0\n";
    return;
  }
  int start = ceil(total / (float(n)));
  for (int i = start; i >= 0; i--) {
    int left = total - i * n;
    ans = min(ans, c * i + d * max(left, 0));
  }
  cout << ans << "\n";
}
int main() {
  ios_base::sync_with_stdio(0);
  cin.tie(0);
  cout.tie(0);
  ;
  int tc = 1;
  while (tc--) {
    solve();
  }
  return 0;
}
