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