#include <bits/stdc++.h>
using namespace std;
int r, c, n, k;
int g[20][20];
int ans = 0;
int main() {
  ios_base ::sync_with_stdio(0);
  memset(g, 0, sizeof(g));
  cin >> r >> c >> n >> k;
  while (n--) {
    int x, y;
    cin >> x >> y;
    g[x][y] = 1;
  }
  for (int i = 1; i <= r; ++i) {
    for (int j = 1; j <= c; ++j) {
      for (int ii = i; ii <= r; ++ii) {
        for (int jj = j; jj <= c; ++jj) {
          int cur = 0;
          for (int k = 1; k <= r; ++k) {
            for (int l = 1; l <= c; ++l) {
              if (g[k][l] == 1 and k <= ii and l <= jj and k >= i and l >= j)
                ++cur;
            }
          }
          if (cur >= k) {
            ++ans;
          }
        }
      }
    }
  }
  cout << ans << endl;
  return 0;
}
