#include <bits/stdc++.h>
using namespace std;
int a[11][11];
int main() {
  int r, c, n, f, x, y;
  cin >> r >> c >> n >> f;
  for (int i = 0; i < n; i++) {
    cin >> x >> y;
    a[x][y] = 1;
  }
  int ans = 0;
  for (int i = 1; i <= r; i++) {
    for (int j = 1; j <= c; j++) {
      for (int u = 1; u <= r; u++) {
        for (int k = 1; k <= c; k++) {
          int cnt = 0;
          for (int st = i; st <= u; st++) {
            for (int en = j; en <= k; en++) {
              if (a[st][en] == 1) cnt++;
            }
          }
          if (cnt >= f) ans++;
        }
      }
    }
  }
  cout << ans;
}
