#include <bits/stdc++.h>
using namespace std;
int a[101][101];
int main() {
  int n, m, i, j, ans = 0, t = 0, l, k, x, y;
  cin >> n >> m >> l >> k;
  for (i = 1; i <= l; i++) {
    cin >> x >> y;
    a[x][y] = 1;
  }
  for (i = 1; i <= n; i++) {
    for (j = 1; j <= m; j++) {
      for (int x1 = i; x1 <= n; x1++) {
        for (int y1 = j; y1 <= m; y1++) {
          int cnt = 0;
          for (int x2 = i; x2 <= x1; x2++) {
            for (int y2 = j; y2 <= y1; y2++) {
              if (a[x2][y2]) cnt++;
            }
          }
          if (k <= cnt) ans++;
        }
      }
    }
  }
  cout << ans;
}
