#include <bits/stdc++.h>
using namespace std;
int main() {
  int a[15][15] = {0}, n, kk, r, c, x, y;
  cin >> r >> c >> n >> kk;
  for (int i = 0; i < n; i++) {
    cin >> x >> y;
    a[x - 1][y - 1] = 1;
  }
  int i, j, k, l, p, q, cnt, ans = 0;
  for (i = 0; i < r; i++)
    for (j = 0; j < c; j++)
      for (k = i; k < r; k++)
        for (l = j; l < c; l++) {
          cnt = 0;
          for (p = i; p <= k; p++) {
            for (q = j; q <= l; q++) cnt += a[p][q];
          }
          if (cnt >= kk) ans++;
        }
  cout << ans << endl;
  return 0;
}
