#include <bits/stdc++.h>
using namespace std;
int arr[15][15];
int sum(int x1, int y1, int x2, int y2) {
  int sum = 0;
  for (; x1 <= x2; x1++)
    for (int y = y1; y <= y2; y++) sum += arr[x1][y];
  return sum;
}
int main() {
  ios::sync_with_stdio(false);
  ios_base::sync_with_stdio(false);
  cin.tie(nullptr), cout.tie(nullptr);
  int n, m, q, k, x, y, counter = 0;
  cin >> n >> m >> q >> k;
  while (q--) cin >> x >> y, arr[x][y] = 1;
  for (int x1 = 1; x1 <= n; x1++)
    for (int y1 = 1; y1 <= m; y1++)
      for (int x2 = x1; x2 <= n; x2++)
        for (int y2 = y1; y2 <= m; y2++)
          if (sum(x1, y1, x2, y2) >= k) counter++;
  cout << counter << endl;
  return 0;
}
