#include <bits/stdc++.h>
using namespace std;
int o, u, risan = 0, i, j, k, l, n, m, x, y, a[15][15] = {0}, cnt = 0;
int main() {
  cin >> n >> m >> x >> y;
  for (i = 0; i < x; i++) {
    cin >> j >> k;
    ++a[j - 1][k - 1];
  }
  for (i = 0; i < n; ++i)
    for (j = i; j < n; ++j)
      for (k = 0; k < m; ++k)
        for (l = k; l < m; ++l) {
          cnt = 0;
          for (o = i; o <= j; o++)
            for (u = k; u <= l; u++)
              if (a[o][u]) ++cnt;
          if (cnt >= y) ++risan;
        }
  cout << risan << endl;
}
