#include <bits/stdc++.h>
using namespace std;
int main() {
  int r, c, n, k, a[12][12];
  cin >> r >> c >> n >> k;
  memset(a, 0, sizeof a);
  for (int i = 0; i < n; i++) {
    int t, tt;
    cin >> t >> tt;
    a[t][tt] = 1;
  }
  int b[12][12];
  memset(b, 0, sizeof b);
  for (int i = 1; i <= r; i++) {
    for (int j = 1; j <= c; j++) {
      b[i][j] = b[i][j - 1] + b[i - 1][j] + a[i][j] - b[i - 1][j - 1];
    }
  }
  int ans = 0;
  for (int i = 1; i <= r; i++) {
    for (int ii = i; ii <= r; ii++) {
      for (int j = 1; j <= c; j++) {
        for (int jj = j; jj <= c; jj++) {
          if (b[ii][jj] - b[i - 1][jj] - b[ii][j - 1] + b[i - 1][j - 1] >= k)
            ans++;
        }
      }
    }
  }
  cout << ans << endl;
}
