#include <bits/stdc++.h>
using namespace std;
struct P {
  int x, y;
} p[10];
int main() {
  int r, c, n, k;
  cin >> r >> c >> n >> k;
  for (int i = 0; i < n; ++i) {
    cin >> p[i].x >> p[i].y;
    --p[i].x;
    --p[i].y;
  }
  int result = 0;
  for (int x = 0; x < r; ++x) {
    for (int y = 0; y < c; ++y) {
      for (int w = 1; w <= r - x; ++w) {
        for (int h = 1; h <= c - y; ++h) {
          int t = 0;
          for (int i = 0; i < n; ++i) {
            if (p[i].x >= x && p[i].x < x + w && p[i].y >= y &&
                p[i].y < y + h) {
              ++t;
            }
          }
          if (t >= k) {
            ++result;
          }
        }
      }
    }
  }
  cout << result << endl;
  return 0;
}
