#include <bits/stdc++.h>
using namespace std;
int const N = 10;
int r, c, n, k;
int a[20][20];
int main() {
  cin >> r >> c >> n >> k;
  for (int i = 0; i < 20; i++) {
    for (int j = 0; j < 20; j++) {
      a[i][j] = 0;
    }
  }
  for (int i = 0; i < n; i++) {
    int x, y;
    cin >> x >> y;
    x--;
    y--;
    a[x][y] = 1;
  }
  int ans = 0;
  for (int x_st = 0; x_st < r; x_st++) {
    for (int x_step = 0; x_st + x_step < r; x_step++) {
      for (int y_st = 0; y_st < c; y_st++) {
        for (int y_step = 0; y_step + y_st < c; y_step++) {
          int sum = 0;
          for (int i = x_st; i <= x_step + x_st; i++) {
            for (int j = y_st; j <= y_step + y_st; j++) {
              sum += a[i][j];
            }
          }
          if (sum >= k) {
            ans++;
          }
        }
      }
    }
  }
  cout << ans << endl;
  return 0;
}
