#include <bits/stdc++.h>
#pragma GCC optimize("O3")
using namespace std;
int main() {
  ios::sync_with_stdio(false);
  cin.tie(0);
  int r, c, n, K, sum = 0;
  cin >> r >> c >> n >> K;
  bool grid[r][c];
  memset(grid, 0, sizeof grid);
  for (int i = 0; i < n; ++i) {
    int x, y;
    cin >> x >> y;
    grid[x - 1][y - 1] = true;
  }
  for (int i = 0; i < r; ++i) {
    for (int j = 0; j < c; ++j) {
      for (int k = i; k < r; ++k) {
        for (int l = j; l < c; ++l) {
          int temp = 0;
          for (int p = i; p <= k; ++p) {
            for (int q = j; q <= l; ++q) {
              if (grid[p][q]) temp++;
            }
          }
          if (temp >= K) sum++;
        }
      }
    }
  }
  cout << sum;
  return 0;
}
