#include <bits/stdc++.h>
using namespace std;
int Map[15][15] = {0}, ro[15] = {0}, co[15] = {0};
int main() {
  int r, c, n, k, r1, c1, i, j, l1, l2, l3, l4, ans = 0, sum = 0;
  scanf("%d%d%d%d", &r, &c, &n, &k);
  for (i = 1; i <= n; i++) {
    scanf("%d%d", &r1, &c1);
    Map[r1][c1] = 1;
  }
  for (i = 1; i <= r; i++) {
    for (j = i; j <= r; j++) {
      for (l1 = 1; l1 <= c; l1++) {
        for (l2 = l1; l2 <= c; l2++) {
          for (l3 = i; l3 <= j; l3++) {
            for (l4 = l1; l4 <= l2; l4++) {
              sum += Map[l3][l4];
            }
          }
          if (sum >= k) {
            ans++;
          }
          sum = 0;
        }
      }
    }
  }
  printf("%d\n", ans);
}
