#include <bits/stdc++.h>
using namespace std;
int rec[15][15];
int main() {
  int r, c, n, k, res = 0;
  cin >> r >> c >> n >> k;
  memset(rec, 0, sizeof(rec));
  for (int m = 0; m < n; m++) {
    int i, j;
    cin >> i >> j;
    rec[i][j] = 1;
  }
  for (int i = 1; i <= r; i++)
    for (int j = 1; j <= c; j++)
      for (int m = i; m <= r; m++)
        for (int n = j; n <= c; n++) {
          int sum = 0;
          for (int k = i; k <= m; k++)
            for (int l = j; l <= n; l++)
              if (rec[k][l]) sum++;
          if (sum >= k) res++;
        }
  cout << res << endl;
}
