#include <bits/stdc++.h>
using namespace std;
int main() {
  int r, c, n, k, x, y, res = 0;
  cin >> r >> c >> n >> k;
  vector<vector<bool> > v(c);
  for (int i = 0; i < c; ++i) v[i].resize(r);
  for (int i = 0; i < n; ++i) {
    cin >> x >> y;
    v[y - 1][x - 1] = true;
  }
  for (int y0 = 0; y0 < c; ++y0) {
    for (int x0 = 0; x0 < r; ++x0) {
      for (int y1 = y0 + 1; y1 <= c; ++y1) {
        for (int x1 = x0 + 1; x1 <= r; ++x1) {
          int cnt = 0;
          for (int i = y0; i < y1; ++i) {
            for (int j = x0; j < x1; ++j) {
              cnt += !!v[i][j];
            }
          }
          if (cnt >= k) res++;
        }
      }
    }
  }
  cout << res << endl;
  return 0;
}
