#include <bits/stdc++.h>
using namespace std;
const int inf = (int)1e9, maxn = (int)1e5 + 1;
const double eps = (double)1e-8;
const int mod = (int)1000000009;
int main() {
  long long n, m, c, d, k, q, o = 0, kol = 0;
  cin >> c >> d >> n >> m >> k;
  q = n;
  n = n * m - k;
  m = q;
  kol = 0;
  while (o < n) {
    if (d * m < c || (d * (n - o) < c)) {
      o++;
      kol += d;
    } else {
      o += m;
      kol += c;
    }
  }
  cout << kol;
  return 0;
}
