#include <bits/stdc++.h>
using namespace std;
int main() {
  ios::sync_with_stdio(false);
  long long r, c, n, k;
  cin >> r >> c >> n >> k;
  vector<long long> x(n);
  vector<long long> y(n);
  for (long long(i) = 0; (i) < (n); ++(i)) {
    cin >> x[i] >> y[i];
    --x[i];
    --y[i];
  }
  long long ans = 0;
  for (long long(x1) = 0; (x1) < (r); ++(x1)) {
    for (long long(y1) = 0; (y1) < (c); ++(y1)) {
      for (long long(x2) = 0; (x2) < (r); ++(x2)) {
        for (long long(y2) = 0; (y2) < (c); ++(y2)) {
          long long q = 0;
          if (x1 <= x2 && y1 <= y2) {
            for (long long(t) = 0; (t) < (n); ++(t)) {
              if (x1 <= x[t] && x2 >= x[t] && y1 <= y[t] && y2 >= y[t]) {
                ++q;
              }
            }
          }
          if (q >= k) {
            ++ans;
          }
        }
      }
    }
  }
  cout << ans;
  return 0;
}
