#include <bits/stdc++.h>
using namespace std;
void print(vector<vector<int> >& b) {
  for (int i = 0; i < b.size(); i++) {
    for (int j = 0; j < b[0].size(); j++) {
      cout << b[i][j] << " ";
    }
    cout << endl;
  }
}
int main() {
  int r, c, n, k;
  cin >> r >> c >> n >> k;
  vector<vector<int> > board(r, vector<int>(c, 0));
  vector<vector<int> > board_int(r, vector<int>(c, 0));
  int count = 0;
  for (int i = 0; i < n; i++) {
    int l, m;
    cin >> l >> m;
    board[l - 1][m - 1] = 1;
  }
  for (int i = 0; i < r; i++) {
    for (int j = 0; j < c; j++) {
      int u = 0, l = 0, d = 0;
      if (j != 0) l = board_int[i][j - 1];
      if (i != 0) u = board_int[i - 1][j];
      if ((i != 0) && (j != 0)) d = board_int[i - 1][j - 1];
      board_int[i][j] = l + u - d + board[i][j];
    }
  }
  for (int st_r = 0; st_r < r; st_r++) {
    for (int st_c = 0; st_c < c; st_c++) {
      for (int end_r = st_r; end_r < r; end_r++) {
        for (int end_c = st_c; end_c < c; end_c++) {
          int br = board_int[end_r][end_c], ur = 0, bl = 0, ul = 0;
          if (st_r != 0) ur = board_int[st_r - 1][end_c];
          if (st_c != 0) bl = board_int[end_r][st_c - 1];
          if (st_r != 0 && st_c != 0) ul = board_int[st_r - 1][st_c - 1];
          int sum = br - ur - bl + ul;
          if (sum >= k) {
            count++;
          }
        }
      }
    }
  }
  cout << count << endl;
  return 0;
}
