#include <bits/stdc++.h>
using namespace std;
int main() {
  ios_base::sync_with_stdio(0);
  int r, c, n, k;
  cin >> r >> c >> n >> k;
  int tab[r][c];
  for (int i = 0; i < r; i++)
    for (int j = 0; j < c; j++) tab[i][j] = 0;
  for (int i = 0; i < n; i++) {
    int x, y;
    cin >> x >> y;
    --x;
    --y;
    tab[x][y] = 1;
  }
  int wyn = 0;
  for (int x1 = 0; x1 < r; x1++)
    for (int y1 = 0; y1 < c; y1++)
      for (int x2 = x1; x2 < r; x2++)
        for (int y2 = y1; y2 < c; y2++) {
          int v = 0;
          for (int i = x1; i <= x2; i++)
            for (int j = y1; j <= y2; j++) v += tab[i][j];
          if (v >= k) wyn++;
        }
  cout << wyn;
}
