#include <bits/stdc++.h>
using namespace std;
void solve();
long long int c, d, n, m, k;
const long long int N = 1e5;
vector<long long int> dp(N, -1);
long long int sol(long long int rem) {
  if (rem <= 0) {
    return 0;
  }
  if (dp[rem] != -1) {
    return dp[rem];
  }
  long long int op1 = c + sol(rem - n);
  long long int op2 = d + sol(rem - 1);
  return dp[rem] = min(op1, op2);
}
int32_t main() {
  ios_base::sync_with_stdio(0);
  cin.tie(0);
  cout.tie(0);
  long long int t = 1;
  while (t--) solve();
  return 0;
}
void solve() {
  long long int i, j;
  cin >> c >> d >> n >> m >> k;
  long long int remain = n * m - k;
  cout << sol(remain) << "\n";
}
