#include <bits/stdc++.h>
using namespace std;
int main() {
  int r, c, n, k, a, b;
  cin >> r >> c >> n >> k;
  vector<vector<int> > ar(r, vector<int>(c, 0));
  for (int i = 0; i < n; i++) {
    cin >> a >> b;
    ar[a - 1][b - 1] = 1;
  }
  int ans = 0, temp;
  for (int i1 = 0; i1 < r; i1++) {
    for (int i2 = 0; i2 < i1 + 1; i2++) {
      for (int j1 = 0; j1 < c; j1++) {
        for (int j2 = 0; j2 < j1 + 1; j2++) {
          temp = 0;
          for (int x = i2; x < i1 + 1; x++) {
            for (int y = j2; y < j1 + 1; y++) {
              if (ar[x][y]) temp++;
            }
          }
          if (temp >= k) ans++;
        }
      }
    }
  }
  cout << ans;
}
