#include <bits/stdc++.h>
using namespace std;
void solve() {
  int a, b, c, d, n, m, k;
  cin >> c >> d >> n >> m >> k;
  a = n * m - k;
  if (a <= 0) {
    cout << 0;
    return;
  }
  if (d * n <= c) {
    cout << (a)*d;
  } else {
    b = a / n;
    b = b * c;
    if (a % n) {
      b += min(c, (a % n) * d);
    }
    cout << b << endl;
  }
}
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
  ;
  solve();
}
