#include <bits/stdc++.h>
using namespace std;
void solve() {
  long long r, c, n, k;
  cin >> r >> c >> n >> k;
  vector<int> x(n), y(n);
  for (int(i) = 0; (i) < (n); ++(i)) cin >> x[i] >> y[i];
  vector<vector<bool> > m(r + 1, vector<bool>(c + 1, false));
  for (int(i) = 0; (i) < (n); ++(i)) {
    m[x[i]][y[i]] = true;
  }
  long long ans = 0;
  for (int(szy) = (1); (szy) < (c + 1); ++(szy))
    for (int(szx) = 0; (szx) < (r + 1); ++(szx)) {
      if (szx == 0) continue;
      for (int(sx) = 0; (sx) < (r + 1); ++(sx)) {
        if (sx == 0) continue;
        for (int(sy) = 0; (sy) < (c + 1); ++(sy)) {
          if (sy == 0) continue;
          if (sx + szx - 1 > r || sy + szy - 1 > c) break;
          int ct = 0;
          for (int(px) = (sx); (px) < (sx + szx); ++(px)) {
            for (int(py) = (sy); (py) < (sy + szy); ++(py)) {
              if (m[px][py]) ++ct;
            }
          }
          if (ct >= k) ++ans;
        }
      }
    }
  cout << ans;
}
int main() {
  ios_base::sync_with_stdio(false);
  solve();
  return 0;
}
