#include <bits/stdc++.h>
using namespace std;
int dp[12][12];
int main() {
  int r, c, n, k, i, j, p, m, l, q, s;
  cin >> r >> c >> n >> k;
  int x, y;
  vector<string> z;
  string qq(c, '*');
  for (i = 0; i < r; ++i) z.push_back(qq);
  for (i = 0; i < n; ++i) {
    cin >> x >> y;
    z[x - 1][y - 1] = '#';
  }
  int ans = 0;
  for (i = 0; i < r; ++i)
    for (j = 0; j < c; ++j)
      for (m = i; m < r; ++m)
        for (l = j; l < c; ++l) {
          int cnt = 0;
          for (q = i; q <= m; ++q)
            for (s = j; s <= l; ++s)
              if (z[q][s] == '#') cnt++;
          if (cnt >= k) ans++;
        }
  cout << ans;
  return 0;
}
