#include <bits/stdc++.h>
using namespace std;
template <class T>
void read(T& arr) {
  for (long long i = 0; i < arr.size(); ++i) cin >> arr[i];
}
long long binpow(long long a, int n) {
  if (n == 0) return 1;
  if (n % 2 == 1)
    return binpow(a, n - 1) * a;
  else {
    long long b = binpow(a, n / 2);
    return b * b;
  }
}
bool check(vector<vector<int> >& v, int x1, int y1, int x2, int y2, int k) {
  int c = 0;
  for (int i = x1; i <= x2; ++i) {
    for (int j = y1; j <= y2; ++j) {
      if (v[i][j] == 1) ++c;
    }
  }
  if (c >= k) return true;
  return false;
}
void solve() {
  int r, c, n, k;
  cin >> r >> c >> n >> k;
  vector<vector<int> > v(r, vector<int>(c, 0));
  for (int i = 0; i < n; ++i) {
    int x, y;
    cin >> x >> y;
    --x, --y;
    v[x][y] = 1;
  }
  int ans = 0;
  for (int a = 0; a < r; ++a) {
    for (int b = 0; b < c; ++b) {
      for (int d = a; d < r; ++d) {
        for (int e = b; e < c; ++e) {
          if (check(v, a, b, d, e, k)) ++ans;
        }
      }
    }
  }
  cout << ans;
}
int main() {
  ios_base::sync_with_stdio(false);
  solve();
  return 0;
}
