#include <bits/stdc++.h>
using namespace std;
template <class C>
constexpr int sz(const C &c) {
  return int(c.size());
}
using ll = long long;
constexpr const char nl = '\n', sp = ' ';
template <class C>
void partial_sum_2d(const C &A, int N, int M, C &pre) {
  for (int i = 0; i < N; i++)
    for (int j = 0; j < M; j++) {
      pre[i][j] = A[i][j];
      if (i > 0) pre[i][j] += pre[i - 1][j];
      if (j > 0) pre[i][j] += pre[i][j - 1];
      if (i > 0 && j > 0) pre[i][j] -= pre[i - 1][j - 1];
    }
}
template <class C>
auto rsq(const C &pre, int u, int d, int l, int r) ->
    typename decay<decltype(pre[d][r])>::type {
  typename decay<decltype(pre[d][r])>::type v = pre[d][r];
  if (u > 0) v -= pre[u - 1][r];
  if (l > 0) v -= pre[d][l - 1];
  if (u > 0 && l > 0) v += pre[u - 1][l - 1];
  return v;
}
int main() {
  ios::sync_with_stdio(0);
  cin.tie(0);
  cout.tie(0);
  int R, C, N, K;
  cin >> R >> C >> N >> K;
  vector<vector<int>> A(R, vector<int>(C, 0));
  for (int i = 0; i < N; i++) {
    int x, y;
    cin >> x >> y;
    A[--x][--y]++;
  }
  partial_sum_2d(A, R, C, A);
  int ans = 0;
  for (int u = 0; u < R; u++)
    for (int d = u; d < R; d++)
      for (int l = 0; l < C; l++)
        for (int r = l; r < C; r++) ans += rsq(A, u, d, l, r) >= K;
  cout << ans << nl;
  return 0;
}
