#include <bits/stdc++.h>
int main(int argc, char const *argv[]) {
  int r, c, n, m;
  int count = 0;
  int coor[11][11] = {0};
  std::cin >> r >> c >> n >> m;
  int X, Y;
  for (size_t i = 0; i < n; i++) {
    std::cin >> X >> Y;
    coor[X - 1][Y - 1] = 1;
  }
  int sum = 0;
  for (size_t i = 0; i < r; i++) {
    for (size_t j = 0; j < c; j++) {
      for (size_t l = i; l < r; l++) {
        for (size_t r = j; r < c; r++) {
          sum = 0;
          for (size_t g = i; g <= l; g++) {
            for (size_t f = j; f <= r; f++) {
              sum += coor[g][f];
            }
          }
          if (sum >= m) {
            count++;
          }
        }
      }
    }
  }
  std::cout << count << std::endl;
  return 0;
}
