#include <bits/stdc++.h>
using namespace std;
int m[50][50];
int main() {
  int r, c, n, k, ans = 0;
  cin >> r >> c >> n >> k;
  for (int i = 0; i < n; i++) {
    int a, b;
    cin >> a >> b;
    m[a - 1][b - 1] = 1;
  }
  for (int z = 1; z <= r; z++)
    for (int zz = 1; zz <= c; zz++) {
      for (int i = 0; i < r; i++) {
        for (int j = 0; j < c; j++) {
          int cnt = 0;
          if (i + z <= r && j + zz <= c)
            for (int ii = i; ii < i + z; ii++)
              for (int jj = j; jj < j + zz; jj++) {
                cnt += m[ii][jj];
              }
          if (cnt >= k) ans++;
        }
      }
    }
  cout << ans;
}
