#include <bits/stdc++.h>
using namespace std;
int main() {
  int dem = 0, r, c, n, k, a[15][15] = {0}, ngang[15][15] = {0};
  cin >> r >> c >> n >> k;
  for (int i = 0; i < n; i++) {
    int x, y;
    cin >> x >> y;
    a[x][y] = 1;
  }
  for (int i = 1; i <= r; i++)
    for (int j = 1; j <= c; j++) {
      if (a[i][j] == 1) ngang[i][j]++;
      ngang[i][j] += ngang[i][j - 1];
    }
  for (int i = 1; i <= r; i++)
    for (int j = 1; j <= c; j++)
      for (int i2 = i; i2 <= r; i2++)
        for (int j2 = j; j2 <= c; j2++) {
          int temp = 0;
          for (int t = i; t <= i2; t++)
            temp += (ngang[t][j2] - ngang[t][j - 1]);
          if (temp >= k) dem++;
        }
  cout << dem;
}
