#include <bits/stdc++.h>
using namespace std;
int main() {
  int r, c, n, k;
  cin >> r >> c >> n >> k;
  bool vio[10][10];
  memset(vio, false, sizeof(vio));
  for (int i = 0; i < n; i++) {
    int x, y;
    cin >> x >> y;
    vio[x - 1][y - 1] = true;
  }
  int cals[10][10][10][10];
  memset(cals, 0, sizeof(cals));
  for (int i = 0; i < r; i++) {
    for (int j = 0; j < c; j++) {
      cals[i][j][i][j] = vio[i][j];
      for (int i1 = i; i1 < r; i1++) {
        for (int j1 = j; j1 < c; j1++) {
          int c1 = 0, c2 = 0, c3 = 0;
          if (j1 - 1 >= j) {
            c1 = cals[i][j][i1][j1 - 1];
          }
          if (j1 - 1 >= j && i1 - 1 >= i) {
            c3 = -cals[i][j][i1 - 1][j1 - 1];
          }
          if (i1 - 1 >= i) {
            c2 = cals[i][j][i1 - 1][j1];
          }
          cals[i][j][i1][j1] = c1 + c2 + c3 + vio[i1][j1];
        }
      }
    }
  }
  int ret = 0;
  for (int i = 0; i < r; i++) {
    for (int j = 0; j < c; j++) {
      for (int i1 = i; i1 < r; i1++) {
        for (int j1 = j; j1 < c; j1++) {
          if (cals[i][j][i1][j1] >= k) ret++;
        }
      }
    }
  }
  cout << ret << endl;
  return 0;
}
