#include <bits/stdc++.h>
using namespace std;
const long long BIG = 1000000007;
const double PI = 3.141592653589793238;
template <typename T, typename Q>
T max(T a, Q b) {
  if (a < b) return b;
  return a;
}
template <typename T, typename Q>
T min(T a, Q b) {
  if (a > b) return b;
  return a;
}
long gcd(long a, long b) {
  if (b == 0) {
    return a;
  }
  return gcd(b, a % b);
}
int check(vector<vector<int>> arr, int p, int q, int r, int s) {
  int ans = 0;
  for (int i = p; i < q; i++) {
    for (int j = r; j < s; j++) ans += arr[i][j];
  }
  return ans;
}
void solve() {
  int r, c, n, K;
  cin >> r >> c >> n >> K;
  vector<vector<int>> arr(r, vector<int>(c, 0));
  for (int i = 0; i < n; i++) {
    int p, q;
    cin >> p >> q;
    arr[p - 1][q - 1] = 1;
  }
  int ans = 0;
  for (int i = 0; i < r; i++) {
    for (int j = i + 1; j <= r; j++) {
      for (int k = 0; k < c; k++) {
        for (int l = k + 1; l <= c; l++) {
          if (check(arr, i, j, k, l) >= K) ans++;
        }
      }
    }
  }
  cout << ans << "\n";
}
int main() {
  int t;
  t = 1;
  while (t--) solve();
  return 0;
}
