#include <bits/stdc++.h>
using namespace std;
#pragma GCC optimize("Ofast")
#pragma GCC target("avx,avx2,fma")
template <typename Arg1>
void __f(const char* name, Arg1&& arg1) {
  cout << name << " = " << arg1 << '\n';
}
template <typename Arg1, typename... Args>
void __f(const char* names, Arg1&& arg1, Args&&... args) {
  const char* comma = strchr(names + 1, ',');
  cout.write(names, comma - names) << " = " << arg1 << " ";
  __f(comma + 1, args...);
}
long long int ceil(long long int a, long long int b) { return (a + b - 1) / b; }
void prefix_2d(vector<vector<long long int>>& prefix,
               vector<vector<long long int>> a) {
  long long int n = prefix.size();
  long long int m = prefix[0].size();
  prefix[1][1] = a[1][1];
  for (long long int i = 2; i < m; i++)
    prefix[1][i] = prefix[1][i - 1] + a[1][i];
  for (long long int i = 2; i < n; i++)
    prefix[i][1] = prefix[i - 1][1] + a[i][1];
  for (long long int i = 2; i < n; i++)
    for (long long int j = 2; j < m; j++)
      prefix[i][j] =
          prefix[i - 1][j] + prefix[i][j - 1] - prefix[i - 1][j - 1] + a[i][j];
}
long long int query(vector<vector<long long int>> p, long long int x1,
                    long long int x2, long long int y1, long long int y2) {
  return p[x2][y2] - p[x1 - 1][y2] - p[x2][y1 - 1] + p[x1 - 1][y1 - 1];
}
void solve() {
  long long int r, c, n, kk, temp, ans = 0;
  cin >> r >> c >> n >> kk;
  vector<vector<long long int>> a(r + 1, vector<long long int>(c + 1, 0));
  long long int x, y;
  for (long long int i = 0; i < (long long int)n; i++) {
    cin >> x >> y;
    a[x][y] = 1;
  }
  vector<vector<long long int>> p(r + 1, vector<long long int>(c + 1, 0));
  prefix_2d(p, a);
  for (long long int i = 1; i < (long long int)r + 1; i++) {
    for (long long int j = 1; j < (long long int)c + 1; j++) {
      for (long long int k = i; k < (long long int)r + 1; k++) {
        for (long long int l = j; l < (long long int)c + 1; l++) {
          long long int val = query(p, i, k, j, l);
          if (val >= kk) {
            ans++;
          }
        }
      }
    }
  }
  cout << ans << '\n';
}
int main() {
  ios::sync_with_stdio(0);
  cin.tie(0);
  solve();
  return 0;
}
