#include <bits/stdc++.h>
using namespace std;
int A[15][15], r, c, n, k, res;
int main() {
  scanf("%d %d %d %d", &r, &c, &n, &k);
  for (int i = (0); i < (n); i++) {
    int x, y;
    scanf("%d %d", &x, &y);
    ;
    A[y][x] = 1;
  }
  for (int r1 = (1); r1 < (r + 1); r1++) {
    for (int c1 = (1); c1 < (c + 1); c1++) {
      for (int r2 = (r1); r2 < (r + 1); r2++) {
        for (int c2 = (c1); c2 < (c + 1); c2++) {
          int sum = 0;
          for (int i = (c1); i < (c2 + 1); i++) {
            for (int j = (r1); j < (r2 + 1); j++) {
              if (A[i][j]) sum++;
            }
          }
          if (sum >= k) res++;
        }
      }
    }
  }
  printf("%d\n", res);
}
