#include <bits/stdc++.h>
using namespace std;
int a[19][19], n, m, cnt, k, ans;
int read() {
  int x = 0, f = 1;
  char ch = getchar();
  while (!isdigit(ch)) {
    if (ch == '-') f = -1;
    ch = getchar();
  }
  while (isdigit(ch)) {
    x = x * 10 + ch - '0';
    ch = getchar();
  }
  return x * f;
}
int main() {
  n = read();
  m = read();
  cnt = read();
  k = read();
  for (int i = 1; i <= cnt; i++) {
    a[read()][read()] = 1;
  }
  for (int i = 1; i <= n; i++)
    for (int j = 1; j <= m; j++)
      for (int x = i; x <= n; x++)
        for (int y = j; y <= m; y++) {
          int sum = 0;
          for (int p = i; p <= x; p++)
            for (int q = j; q <= y; q++)
              if (a[p][q]) sum++;
          if (sum >= k) ans++;
        }
  printf("%d\n", ans);
  return 0;
}
