#include <bits/stdc++.h>
using namespace std;
bool within(int x1, int x2, int y1, int y2, int px, int py) {
  if (x1 <= px and px <= x2 and y1 <= py and py <= y2) {
    return true;
  }
  return false;
}
int main() {
  ios::sync_with_stdio(false);
  int y, x, v, n;
  cin >> y >> x >> v >> n;
  int py[v], px[v];
  for (int i = 0; i < v; i++) {
    cin >> px[i] >> py[i];
    py[i]--, px[i]--;
  }
  int res = 0;
  for (int x1 = 0; x1 < x; x1++) {
    for (int x2 = x1; x2 < x; x2++) {
      for (int y1 = 0; y1 < y; y1++) {
        for (int y2 = y1; y2 < y; y2++) {
          int found = 0;
          for (int i = 0; i < v; i++) {
            if (within(x1, x2, y1, y2, py[i], px[i])) {
              found++;
            }
          }
          if (found >= n) {
            res++;
          }
        }
      }
    }
  }
  cout << res;
  return 0;
}
