#include <bits/stdc++.h>
using namespace std;
int c, d, n, m, k, dp[10002];
void pre_process() {
  dp[n] = min(n * d, c);
  for (int i = 0; i < n; i++) dp[i] = min(c, i * d);
}
int saghf(int n, int m) {
  if (n % m == 0) return n / m;
  return (n / m) + 1;
}
int main() {
  cin >> c >> d;
  cin >> n >> m;
  cin >> k;
  int o = m * n - k;
  if (m == 0) {
    cout << 0;
    return 0;
  }
  pre_process();
  for (int i = n + 1; i < o + 1; i++) dp[i] = min(dp[i - n] + c, dp[i - 1] + d);
  cout << dp[o];
  return 0;
}
