#include <bits/stdc++.h>
using namespace std;
const int maxn = 1000 + 10;
int main() {
  int c, d, n, m, k;
  while (scanf("%d %d", &c, &d) != EOF) {
    scanf("%d %d", &n, &m);
    scanf("%d", &k);
    int t = n * m - k;
    if (t <= 0) {
      printf("0\n");
      continue;
    }
    int res = t / n;
    int ans = min(res * n * d, res * c);
    ans += min(c, (t - res * n) * d);
    cout << ans << endl;
  }
  return 0;
}
