#include <bits/stdc++.h>
using namespace std;
int main() {
  ios::sync_with_stdio(false);
  cin.tie(0);
  int n, m, i, j, k, nn, kk, ans = 0, x, y, s, aa, bb;
  cin >> n >> m >> nn >> kk;
  vector<vector<int>> a(n, vector<int>(m, 0));
  for (i = 0; i < nn; i++) {
    cin >> x >> y;
    x--;
    y--;
    a[x][y] = 1;
  }
  ans = 0;
  for (i = 0; i < n; i++) {
    for (j = 0; j < m; j++) {
      for (aa = i; aa < n; aa++) {
        for (bb = j; bb < m; bb++) {
          s = 0;
          for (x = i; x <= aa; x++)
            for (y = j; y <= bb; y++) {
              s += a[x][y];
            }
          if (s >= kk) ans++;
        }
      }
    }
  }
  cout << ans << "\n";
  return 0;
}
