#include <bits/stdc++.h>
int main() {
  static int aa[10][10];
  int r, c, n, k, x1, x2, y1, y2, cnt1;
  scanf("%d%d%d%d", &r, &c, &n, &k);
  while (n--) {
    int x, y;
    scanf("%d%d", &x, &y), x--, y--;
    aa[x][y] = 1;
  }
  cnt1 = 0;
  for (x1 = 0; x1 < r; x1++)
    for (x2 = x1; x2 < r; x2++)
      for (y1 = 0; y1 < c; y1++)
        for (y2 = y1; y2 < c; y2++) {
          int x, y, cnt2;
          cnt2 = 0;
          for (x = x1; x <= x2; x++)
            for (y = y1; y <= y2; y++)
              if (aa[x][y] == 1) cnt2++;
          if (cnt2 >= k) cnt1++;
        }
  printf("%d\n", cnt1);
  return 0;
}
