#include <bits/stdc++.h>
using namespace std;
int r, c, n, k, ans = 0;
int a[111][111];
int main() {
  cin >> r >> c >> n >> k;
  while (n--) {
    int x, y;
    cin >> x >> y;
    a[x][y] = 1;
  }
  for (int q = 1; q <= r; q++)
    for (int w = q; w <= r; w++)
      for (int e = 1; e <= c; e++)
        for (int r = e; r <= c; r++) {
          int cnt = 0;
          for (int i = q; i <= w; i++)
            for (int j = e; j <= r; j++) cnt += a[i][j];
          ans += cnt >= k;
        }
  cout << ans;
}
