#include <bits/stdc++.h>
using namespace std;
int n, r, c, k;
int a[15][15];
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
  cin >> r >> c >> n >> k;
  memset(a, 0, sizeof(a));
  for (int i = 1; i <= n; i++) {
    int x, y;
    cin >> x >> y;
    a[x][y] = 1;
  }
  int ans = 0;
  for (int x = 1; x <= r; x++)
    for (int y = 1; y <= c; y++)
      for (int u = x; u <= r; u++)
        for (int v = y; v <= c; v++) {
          int cnt = 0;
          for (int i = x; i <= u; i++)
            for (int j = y; j <= v; j++)
              if (a[i][j] == 1) cnt++;
          if (cnt >= k) ans++;
        }
  cout << ans << endl;
  return 0;
}
