#include <bits/stdc++.h>
using namespace std;
int main() {
  int cnt = 0;
  int r, c, n, k, rw, cl;
  cin >> r >> c >> n >> k;
  int** orch = new int*[r];
  for (int r_ix = 0; r_ix < r; r_ix++) orch[r_ix] = new int[c];
  for (int r_ix = 0; r_ix < r; r_ix++)
    for (int c_ix = 0; c_ix < c; c_ix++) orch[r_ix][c_ix] = 0;
  for (int n_ix = 0; n_ix < n; n_ix++) {
    cin >> rw >> cl;
    orch[rw - 1][cl - 1] = 1;
  }
  for (int fromr = 0; fromr < r; fromr++) {
    for (int tor = fromr; tor < r; tor++) {
      for (int fromc = 0; fromc < c; fromc++) {
        for (int toc = fromc; toc < c; toc++) {
          int sum = 0;
          for (int is = fromr; is <= tor; is++) {
            for (int js = fromc; js <= toc; js++) {
              sum += orch[is][js];
            }
          }
          if (sum >= k) cnt++;
        }
      }
    }
  }
  cout << cnt;
}
