#include <bits/stdc++.h>
using namespace std;
const int INF = 0x3f3f3f3f;
int main() {
  int c, d, n, m, k;
  int dp[105];
  cin >> c >> d >> n >> m >> k;
  int res = 0;
  int all = n * m - k;
  if (all <= 0) {
    puts("0");
    return 0;
  }
  int p = d * n;
  int t = all / n;
  int g = all % n;
  res = t * min(c, p) + min(g * d, c);
  cout << res << endl;
  return 0;
}
