#include <bits/stdc++.h>
int n, m, a, L, x, y, g[11][11], N, A;
int main() {
  std::cin >> n >> m >> a >> L;
  for (int i = 1; i <= a; i++) std::cin >> x >> y, g[x][y]++;
  for (int i = 1; i <= n; i++)
    for (int j = 1; j <= m; j++)
      for (int k = i; k <= n; k++)
        for (int u = j; u <= m; u++) {
          N = 0;
          for (int s = i; s <= k; s++)
            for (int t = j; t <= u; t++) N += g[s][t];
          A += N >= L;
        }
  std::cout << A;
}
