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