#include <bits/stdc++.h>
using namespace std;
int main() {
  int rows, columns, points, least, t1, t2, out = 0;
  cin >> rows >> columns >> points >> least;
  vector<vector<char>> map(rows);
  for (int i = 0; i < rows; i++) {
    map[i].resize(columns, '.');
  }
  for (int i = 0; i < points; i++) {
    cin >> t1 >> t2;
    map[t1 - 1][t2 - 1] = '#';
  }
  set<pair<pair<int, int>, pair<int, int>>> rectangels;
  for (int startrow = 0; startrow < rows; startrow++) {
    for (int startcol = 0; startcol < columns; startcol++) {
      for (int endrow = startrow; endrow < rows; endrow++) {
        for (int endcol = startcol, hashs = 0; endcol < columns;
             endcol++, hashs = 0) {
          for (int w = min(startrow, endrow); w <= max(startrow, endrow); w++) {
            for (int x = min(startcol, endcol); x <= max(startcol, endcol);
                 x++) {
              if (map[w][x] == '#') {
                hashs++;
              }
            }
          }
          if (hashs >= least) {
            out++;
          }
        }
      }
    }
  }
  cout << out << endl;
  return 0;
}
