#include <bits/stdc++.h>
using namespace std;
bool on[22][22];
int main() {
  int r, c, n, k;
  cin >> r >> c >> n >> k;
  for (int i = 0; i < n; i++) {
    int x, y;
    cin >> x >> y;
    on[x][y] = 1;
  }
  int ans = 0;
  for (int i = 1; i <= r; i++) {
    for (int j = 1; j <= c; j++) {
      for (int ii = i; ii <= r; ii++) {
        for (int jj = j; jj <= c; jj++) {
          int cnt = 0;
          for (int x = i; x <= ii; x++) {
            for (int y = j; y <= jj; y++) {
              if (on[x][y]) cnt++;
            }
          }
          if (cnt >= k) ans++;
        }
      }
    }
  }
  cout << ans << endl;
  return 0;
}
