#include <bits/stdc++.h>
using namespace std;
const int INF = 1000000000;
int n, k, m, c, d, res = 0;
void Run() {
  cin >> c >> d;
  cin >> n >> m;
  cin >> k;
  int v = n * m - k;
  if (c / n <= d) {
    while (v >= n) {
      v -= n;
      res += c;
    }
    if (v != 0 && c < v * d)
      res += c;
    else if (v != 0 && c >= v * d)
      res += v * d;
  } else
    res = v * d;
  if (n * m <= k) res = 0;
  cout << res;
}
int main() {
  ios_base::sync_with_stdio(0);
  Run();
  return 0;
}
