#include <bits/stdc++.h>
using namespace std;
int r, c, n, k, a[12][12], x, y, nr;
int main() {
  cin >> r >> c >> n >> k;
  int i, j, l, m, q, p, s;
  for (i = 1; i <= n; i++) {
    cin >> x >> y;
    a[x][y] = 1;
  }
  for (i = 1; i < r; i++)
    for (j = i + 1; j <= r; j++)
      for (l = 1; l < c; l++)
        for (m = l + 1; m <= c; m++) {
          s = 0;
          for (p = i; p <= j; p++)
            for (q = l; q <= m; q++) s = s + a[p][q];
          if (s >= k) nr++;
        }
  if (k == 1) nr = nr + n;
  for (i = 1; i <= r; i++)
    for (j = 1; j < c; j++)
      for (l = j + 1; l <= c; l++) {
        s = 0;
        for (m = j; m <= l; m++)
          if (a[i][m] == 1) s++;
        if (s >= k) nr++;
      }
  for (i = 1; i <= c; i++)
    for (j = 1; j < r; j++)
      for (l = j + 1; l <= r; l++) {
        s = 0;
        for (m = j; m <= l; m++)
          if (a[m][i] == 1) s++;
        if (s >= k) nr++;
      }
  cout << nr << "\n";
  return 0;
}
