#include <bits/stdc++.h>
const long long int mod = 1000000007;
const int INF = 1000000000;
using namespace std;
unsigned long long int pow2(unsigned long long int s) {
  if (s != 0 && (s & (s - 1)))
    return 0;
  else
    return 1;
}
unsigned long long int powany(long long int a, long long int b) {
  unsigned long long int res = 1;
  a %= mod;
  for (; b; b >>= 1) {
    if (b & 1) res = res * a % mod;
    a = a * a % mod;
  }
  return res;
}
unsigned long long int prime(unsigned long long int n) {
  if (n <= 1) return 0;
  if (n <= 3) return 1;
  if (n % 2 == 0 || n % 3 == 0) return 0;
  for (unsigned long long int i = 5; i * i <= n; i += 6)
    if (n % (i) == 0 || n % (i + 2) == 0) return 0;
  return 1;
}
int main() {
  int n, m, k, x, y, l = 0, s = 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;
          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;
}
