#include <bits/stdc++.h>
int search(int arr[], int n, int x);
void mergeSort(int arr[], int l, int r);
void merge(int arr[], int l, int m, int r);
int main(void) {
  int n, d;
  scanf("%i %i", &n, &d);
  int arr[100];
  for (int i = 0; i < n; i++) {
    scanf("%i", &arr[i]);
  }
  int m;
  scanf("%i", &m);
  mergeSort(arr, 0, n - 1);
  int profit = 0;
  int n_track = 0;
  for (int i = 0; i < m; i++) {
    if (i >= n) {
      profit -= d;
    } else {
      profit += arr[n_track++];
    }
  }
  printf("\n%i", profit);
}
int search(int arr[], int n, int x) {
  int i;
  for (i = 0; i < n; i++)
    if (arr[i] == x) return i;
  return -1;
}
void merge(int arr[], int l, int m, int r) {
  int i, j, k;
  int n1 = m - l + 1;
  int n2 = r - m;
  int L[n1], R[n2];
  for (i = 0; i < n1; i++) L[i] = arr[l + i];
  for (j = 0; j < n2; j++) R[j] = arr[m + 1 + j];
  i = 0;
  j = 0;
  k = l;
  while (i < n1 && j < n2) {
    if (L[i] <= R[j]) {
      arr[k] = L[i];
      i++;
    } else {
      arr[k] = R[j];
      j++;
    }
    k++;
  }
  while (i < n1) {
    arr[k] = L[i];
    i++;
    k++;
  }
  while (j < n2) {
    arr[k] = R[j];
    j++;
    k++;
  }
}
void mergeSort(int arr[], int l, int r) {
  if (l < r) {
    int m = l + (r - l) / 2;
    mergeSort(arr, l, m);
    mergeSort(arr, m + 1, r);
    merge(arr, l, m, r);
  }
}
