#include <bits/stdc++.h>
using namespace std;
int mas[10][10];
int summ(int x1, int y1, int x2, int y2) {
  int r = 0;
  for (int z = x1; z <= x2; ++z) {
    for (int c = y1; c <= y2; ++c) {
      r += mas[z][c];
    }
  }
  return r;
}
int main() {
  int r, c, n, k;
  cin >> r >> c >> n >> k;
  for (int i = 0; i < n; ++i) {
    int x, y;
    cin >> x >> y;
    mas[x - 1][y - 1] = 1;
  }
  int res = 0;
  for (int x1 = 0; x1 < r; ++x1) {
    for (int y1 = 0; y1 < c; ++y1) {
      for (int x2 = x1; x2 < r; ++x2) {
        for (int y2 = y1; y2 < c; ++y2) {
          if (summ(x1, y1, x2, y2) >= k) {
            res++;
          }
        }
      }
    }
  }
  cout << res;
  return 0;
}
