#include <bits/stdc++.h>
using namespace std;
auto clk = clock();
vector<long long> dp(100000);
void solve() {
  long long c, d;
  cin >> c >> d;
  long long n, m;
  cin >> n >> m;
  long long k;
  cin >> k;
  long long e = (n * m) - k;
  for (long long i = 1; i < e + 1; i++) {
    dp[i] = min(dp[i - 1] + d, dp[max(0ll, i - n)] + c);
  }
  cout << dp[max(0ll, e)] << "\n";
}
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
  cout.tie(NULL);
  cout << fixed << setprecision(8);
  long long t = 1;
  while (t--) solve();
  return 0;
}
