#include <bits/stdc++.h>
using namespace std;
int r, c, n, k;
int a[11][11];
int main() {
  memset(a, 0, sizeof(a));
  cin >> r >> c >> n >> k;
  int x, y;
  int ans = 0;
  for (int i = 0; i < n; i++) {
    cin >> x >> y;
    a[x][y] = 1;
  }
  for (int i = 1; i <= r; i++) {
    for (int j = 1; j <= c; j++) {
      for (int kk = i; kk <= r; kk++) {
        for (int l = j; l <= c; l++) {
          int tmp = 0;
          for (int x = i; x <= kk; x++) {
            for (int y = j; y <= l; y++) {
              if (a[x][y] == 1) tmp++;
            }
          }
          if (tmp >= k) ans++;
        }
      }
    }
  }
  cout << ans << endl;
}
