#include <bits/stdc++.h>
using namespace std;
bool T[11][11];
int n, m, c, k;
bool check(int a, int b, int c, int d) {
  if (a + c > n || b + d > m) return false;
  long long maxi = 0;
  for (int i = a; i <= a + c; i++)
    for (int j = b; j <= b + d; j++)
      if (T[i][j]) maxi++;
  if (maxi >= k)
    return true;
  else
    return false;
}
int main() {
  scanf("%d %d %d %d", &n, &m, &c, &k);
  for (int i = 0; i < c; i++) {
    int a, b;
    scanf("%d %d", &a, &b);
    T[a][b] = true;
  }
  long long ans = 0;
  for (int l = 0; l <= 10; l++)
    for (int i = 0; i <= 10; i++)
      for (int j = 1; j <= n; j++)
        for (int k = 1; k <= m; k++)
          if (check(j, k, i, l)) ans++;
  printf("%I64d", ans);
  return 0;
}
