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