#include <bits/stdc++.h>
using namespace std;
int r, c, n, k, x, y, ans, a[11][11], j2;
int main() {
  cin >> r >> c >> n >> k;
  for (int i = 0; i < n; i++) cin >> x >> y, a[x][y] = 1;
  for (int i = 1; i <= r; i++)
    for (int u = i; u <= r; u++)
      for (int j = 1; j <= c; j++)
        for (int k2 = j; k2 <= c; k2++) {
          j2 = 0;
          for (int i2 = i; i2 <= u; i2++)
            for (int u2 = j; u2 <= k2; u2++) j2 += a[i2][u2];
          ans += j2 >= k;
        }
  cout << ans;
  return 0;
}
