#include <bits/stdc++.h>
long countViolas(std::vector<std::pair<int, int> > v, int x0, int x1, int y0,
                 int y1) {
  long res(0);
  for (int p = 0; p < v.size(); p++) {
    res += (x0 <= v[p].first && v[p].first <= x1 && y0 <= v[p].second &&
            v[p].second <= y1);
  }
  return res;
}
int main() {
  int r, c, n, k;
  std::cin >> r >> c >> n >> k;
  std::vector<std::pair<int, int> > violas(n);
  for (int p = 0; p < n; p++) {
    std::cin >> violas[p].first >> violas[p].second;
  }
  long count(0);
  for (int xs = 1; xs <= r; xs++) {
    for (int xf = xs; xf <= r; xf++) {
      for (int ys = 1; ys <= c; ys++) {
        for (int yf = ys; yf <= c; yf++) {
          if (countViolas(violas, xs, xf, ys, yf) >= k) {
            ++count;
          }
        }
      }
    }
  }
  std::cout << count << std::endl;
  return 0;
}
