#include <bits/stdc++.h>
using namespace std;
const long long mod = 1e9 + 7;
const long long inf = 922337203685477;
const long long mininf = -922337203685477;
const long long nax = 0;
long long c, d, n, m, k, tot;
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
  cout.tie(NULL);
  cin >> c >> d >> n >> m >> k;
  tot = n * m - k;
  long long ans = 0;
  while (tot > 0) {
    if (c <= d * min(tot, n)) {
      tot -= n;
      ans += c;
    } else {
      ans += d * min(tot, n);
      tot -= min(tot, n);
    }
  }
  cout << ans << '\n';
}
