#include <bits/stdc++.h>
using namespace std;
int main() {
  int c, d, n, m, k;
  cin >> c >> d >> n >> m >> k;
  int users = n * m - k;
  int ans = 1000000000;
  for (int i = 0; i <= m; i++)
    for (int j = 0; j <= 10000; j++) {
      if (i * n + j >= users) {
        if (c * i + j * d < ans) ans = c * i + j * d;
      }
    }
  cout << ans;
}
