#include <bits/stdc++.h>
using namespace std;
int n, m, q, k, i, j, mark[15][15], answ;
int main() {
  cin >> n >> m >> q >> k;
  while (q--) {
    int a, b;
    cin >> a >> b;
    mark[a][b] = 1;
  }
  for (i = 1; i <= n; i++)
    for (j = 1; j <= m; j++) {
      for (int ii = i; ii <= n; ii++)
        for (int jj = j; jj <= m; jj++) {
          int kk = 0;
          for (int a = i; a <= ii; a++)
            for (int b = j; b <= jj; b++) {
              if (mark[a][b]) kk++;
            }
          if (kk >= k) answ++;
        }
    }
  cout << answ << endl;
  return 0;
}
