#include <bits/stdc++.h>
using namespace std;
int c, d, n, m, k;
int main() {
  cin >> c >> d;
  cin >> n >> m;
  cin >> k;
  int sum = m * n;
  sum -= k;
  float a = (float)(c / n);
  if (sum < 0) {
    cout << 0 << endl;
    return 0;
  }
  if (a >= d) {
    cout << sum * d << endl;
  } else {
    int a1 = sum / n;
    int s1 = (a1 + 1) * c;
    int s2 = (a1 * c) + (sum - a1 * n) * d;
    int ans = s1 > s2 ? s2 : s1;
    cout << ans << endl;
  }
  return 0;
}
