#include <bits/stdc++.h>
using namespace std;
int main() {
  long long n, m, c, d, k;
  cin >> c >> d;
  cin >> n >> m >> k;
  long long need = n * m - k;
  long long minimum = 99999999;
  for (long long i = 0; i < 10000; i++) {
    long long rec = (need - i * n);
    if (rec < 0) rec = 0;
    minimum = min(minimum, c * i + rec * d);
  }
  cout << minimum;
  return 0;
}
