#include <bits/stdc++.h>
using namespace std;
vector<pair<int, int> > v;
int main() {
  ::ios_base::sync_with_stdio(false);
  cin.tie(0);
  cout.tie(0);
  int r, c, n, k;
  cin >> r >> c >> n >> k;
  v.resize(n);
  for (int i = 0; i < n; i++) {
    cin >> v[i].first >> v[i].second;
    --v[i].first;
    --v[i].second;
  }
  int ans = 0;
  for (int x1 = 0; x1 < r; x1++)
    for (int x2 = x1; x2 < r; x2++)
      for (int y1 = 0; y1 < c; y1++)
        for (int y2 = y1; y2 < c; y2++) {
          int cnt = 0;
          for (auto u : v) {
            if (u.first >= x1 && u.first <= x2)
              if (u.second >= y1 && u.second <= y2) ++cnt;
          }
          ans += cnt >= k;
        }
  cout << ans;
  return 0;
}
