#include <bits/stdc++.h>
using namespace std;
int main() {
  int r, c, n, k;
  cin >> r >> c >> n >> k;
  bool arr[15][15];
  for (int i = 0; i < 15; i++) {
    for (int j = 0; j < 15; j++) arr[i][j] = 0;
  }
  for (int i = 0; i < n; i++) {
    int x, y;
    cin >> x >> y;
    arr[x][y] = 1;
  }
  bool ok;
  int counter = 0;
  for (int sz1 = 0; sz1 < r; sz1++)
    for (int sz2 = 0; sz2 < c; sz2++)
      for (int i = 1; i <= r; i++) {
        for (int j = 1; j <= c; j++) {
          int temp = 0;
          for (int p = i; p <= i + sz1 && i + sz1 <= r; p++) {
            for (int t = j; t <= j + sz2 && j + sz2 <= c; t++) {
              if (arr[p][t] == 1) temp++;
            }
          }
          if (temp >= k) counter++;
        }
      }
  cout << counter;
  return 0;
}
