#include <bits/stdc++.h>
using namespace std;
long long i, j, k, l, ii, t, m, h, s, n, a[311111], b, c, d, e;
string p;
char q, r;
map<long long, long long> y;
int main() {
  cin >> c >> d;
  cin >> n >> m;
  cin >> k;
  if (k >= n * m) {
    cout << 0;
    return 0;
  }
  k = n * m - k;
  s = 1111111;
  t = min((k + n - 1) / n * c, k * d);
  t = min(t, k / n * c + k % n * d);
  cout << t;
}
