#include <bits/stdc++.h>
using namespace std;
vector<pair<int, int> > v;
int main() {
  int r, c, n, k;
  cin >> r >> c >> n >> k;
  for (int i = 0; i < n; i++) {
    int x, y;
    cin >> x >> y;
    x--;
    y--;
    v.push_back(make_pair(x, y));
  }
  int ats = 0;
  for (int kr = 0; kr < r; kr++) {
    for (int kc = 0; kc < c; kc++) {
      for (int kw = kr; kw < r; kw++) {
        for (int kh = kc; kh < c; kh++) {
          int cur = 0;
          for (int i = 0; i < v.size(); i++) {
            if (v[i].first >= kr && v[i].first <= kw && v[i].second >= kc &&
                v[i].second <= kh)
              cur++;
          }
          if (cur >= k) ats++;
        }
      }
    }
  }
  cout << ats << endl;
}
