#include <bits/stdc++.h>
using namespace std;
char s[15][15];
int a[15][15];
int func(int i1, int j1, int i2, int j2) {
  int i, j, ret = 0;
  for (i = i1; i <= i2; i++) {
    for (j = j1; j <= j2; j++) {
      if (a[i][j] == 1) ret++;
    }
  }
  return ret;
}
int main() {
  int i, j, k, l, m, n, r, c, p, q, ans = 0, i1, i2, j1, j2;
  scanf("%d %d %d %d", &r, &c, &n, &k);
  for (i = 1; i <= n; i++) {
    scanf("%d %d", &p, &q);
    a[p][q] = 1;
  }
  for (i1 = 1, ans = 0; i1 <= r; i1++) {
    for (j1 = 1; j1 <= c; j1++) {
      for (i2 = i1; i2 <= r; i2++) {
        for (j2 = j1; j2 <= c; j2++) {
          l = func(i1, j1, i2, j2);
          if (l >= k) ans++;
        }
      }
    }
  }
  printf("%d\n", ans);
  return 0;
}
