#include <bits/stdc++.h>
using namespace std;
bool isprime(unsigned long long n) {
  if (n <= 1) return false;
  if (n <= 3) return true;
  if (n % 2 == 0 || n % 3 == 0) return false;
  for (unsigned long long i = 5; i * i <= n; i = i + 6)
    if (n % i == 0 || n % (i + 2) == 0) return false;
  return true;
}
int main() {
  int n, m, k, x, y, l = 0, s = 0, t(0);
  cin >> n >> m >> k >> l;
  char ar[20][20];
  for (int i = 0; i < 20; i++)
    for (int j = 0; j < 20; j++) ar[i][j] = '*';
  int arr[20] = {0}, a[20] = {0};
  for (int i = 0; i < k; ++i) {
    cin >> x >> y;
    ar[x - 1][y - 1] = '#';
  }
  for (int i = 0; i < n; ++i) {
    for (int u = 0; u < m; u++) a[u] = 0;
    for (int j = i; j < n; j++) {
      for (int p = 0; p < m; p++) {
        if (ar[j][p] == '#') {
          a[p] += 1;
          t++;
          int d = 0;
          for (int r = 0; r <= p; r++) {
            d = 0;
            for (int y = r; y <= p; y++) d += a[y];
            if (d >= l) {
              s++;
            }
          }
        } else {
          int d = 0;
          for (int r = 0; r <= p; r++) {
            d = 0;
            for (int y = r; y <= p; y++) d += a[y];
            if (d >= l) {
              s++;
            }
          }
        }
      }
    }
  }
  cout << s;
  return 0;
}
