#include <bits/stdc++.h>
using namespace std;
int q[100][100] = {
    0,
};
int bum(int x, int y, int z, int u) {
  int kol = 0;
  for (int i = x; i <= z; i++)
    for (int j = y; j <= u; j++)
      if (q[i][j] == 1) kol++;
  return kol;
}
int main() {
  int r, c, n, k, t, y, u, x, time = 0, g, h, z;
  cin >> r >> c >> n >> k;
  z = k;
  for (int i = 0; i < n * 2; i = i + 2) {
    cin >> x >> y;
    q[x - 1][y - 1] = 1;
  }
  for (int i = 0; i < r; i++)
    for (int j = 0; j < c; j++)
      for (int g = i; g < r; g++)
        for (int h = j; h < c; h++) {
          if (bum(i, j, g, h) >= k) time++;
        }
  cout << time;
  return 0;
}
