#include <bits/stdc++.h>
using namespace std;
int main() {
  int r, c, n, q;
  int a[15][15] = {0};
  cin >> r >> c >> n >> q;
  int res = 0;
  for (int i = 0; i < n; i++) {
    int x, y;
    cin >> x >> y;
    a[x][y] = 1;
  }
  for (int i = 1; i <= r; i++) {
    for (int j = 1; j <= c; j++) {
      a[i][j] = a[i][j] + a[i - 1][j] + a[i][j - 1] - a[i - 1][j - 1];
    }
  }
  for (int i = 1; i <= r; i++) {
    for (int j = i; j <= r; j++) {
      for (int k = 1; k <= c; k++) {
        for (int l = k; l <= c; l++) {
          if (a[j][l] - a[i - 1][l] - a[j][k - 1] + a[i - 1][k - 1] >= q) {
            res++;
          }
        }
      }
    }
  }
  cout << res;
}
