#include <bits/stdc++.h>
using namespace std;
const double pi = acos(-1);
int main() {
  cin.tie(0);
  std::ios::sync_with_stdio(false);
  int r, c, n, k;
  cin >> r >> c >> n >> k;
  int a[r][c];
  for (__typeof(r) i = (0) - ((0) > (r)); i != (r) - ((0) > (r));
       i += 1 - 2 * ((0) > (r)))
    for (__typeof(c) j = (0) - ((0) > (c)); j != (c) - ((0) > (c));
         j += 1 - 2 * ((0) > (c)))
      a[i][j] = 0;
  while (n--) {
    int x, y;
    cin >> x >> y;
    a[x - 1][y - 1] = 1;
  }
  long long ans = 0;
  for (__typeof(r) I = (0) - ((0) > (r)); I != (r) - ((0) > (r));
       I += 1 - 2 * ((0) > (r))) {
    for (__typeof(c) J = (0) - ((0) > (c)); J != (c) - ((0) > (c));
         J += 1 - 2 * ((0) > (c))) {
      long long tmp = 0;
      for (__typeof(c) K = (J) - ((J) > (c)); K != (c) - ((J) > (c));
           K += 1 - 2 * ((J) > (c))) {
        for (__typeof(r) i = (I) - ((I) > (r)); i != (r) - ((I) > (r));
             i += 1 - 2 * ((I) > (r))) {
          for (__typeof(K + 1) j = (J) - ((J) > (K + 1));
               j != (K + 1) - ((J) > (K + 1)); j += 1 - 2 * ((J) > (K + 1)))
            tmp += a[i][j];
          if (tmp >= k) ans++;
        }
        tmp = 0;
      }
    }
  }
  cout << ans;
}
