#include <bits/stdc++.h>
using namespace std;
int main() {
  int r, c, n, k;
  cin >> r >> c >> n >> k;
  int x[n], y[n], res = 0, xi, yi;
  for (int i = 0; i < n; i++) {
    cin >> xi >> yi;
    x[i] = xi - 1;
    y[i] = yi - 1;
  }
  for (int i = 0; i < r; i++) {
    for (int j = 0; j < c; j++) {
      for (int i2 = i; i2 < r; i2++) {
        for (int j2 = j; j2 < c; j2++) {
          int violasenimg = 0;
          for (int v = 0; v < n; v++) {
            if (i <= x[v] && x[v] <= i2 && j <= y[v] && y[v] <= j2)
              violasenimg++;
          }
          if (violasenimg >= k) res++;
        }
      }
    }
  }
  cout << res << endl;
}
