#include <bits/stdc++.h>
using namespace std;
ifstream in("in.txt");
int m[110][110];
int main() {
  int r, c, n, k, result = 0;
  cin >> r >> c >> n >> k;
  for (int i = 0; i < n; i++) {
    int x, y;
    cin >> x >> y;
    m[x - 1][y - 1] = 1;
  }
  for (int l1 = 1; l1 <= r; l1++) {
    for (int l2 = 1; l2 <= c; l2++) {
      for (int a = 0; a + l1 - 1 < r; a++) {
        for (int b = 0; b + l2 - 1 < c; b++) {
          int x = 0;
          for (int i = a; i < a + l1; ++i)
            for (int j = b; j < b + l2; ++j) x += m[i][j];
          if (x >= k) result++;
        }
      }
    }
  }
  cout << result << endl;
  return 0;
}
