#include <bits/stdc++.h>
using namespace std;
int main() {
  int r, c, n, k, i, j, b, d, e, f, count1 = 0, count2 = 0;
  char a[12][12];
  scanf("%d%d%d%d", &r, &c, &n, &k);
  for (i = 1; i <= r; i++) {
    for (j = 1; j <= c; j++) {
      a[i][j] = '*';
    }
  }
  for (b = 0; b < n; b++) {
    scanf("%d%d", &i, &j);
    a[i][j] = '#';
  }
  for (i = 1; i <= r; i++) {
    for (j = 1; j <= c; j++) {
      for (b = i; b <= r; b++) {
        for (d = j; d <= c; d++) {
          count1 = 0;
          for (e = i; e <= b; e++) {
            for (f = j; f <= d; f++) {
              if (a[e][f] == '#') count1++;
            }
          }
          if (count1 >= k) count2++;
        }
      }
    }
  }
  printf("%d\n", count2);
  return 0;
}
