#include <bits/stdc++.h>
using namespace std;
const int MAX = 11;
bool t[MAX][MAX];
int r, c, n, k;
int main() {
  ios_base::sync_with_stdio(0);
  cin.tie(0);
  cin >> r >> c >> n >> k;
  while (n--) {
    int x, y;
    cin >> x >> y;
    t[x - 1][y - 1] = 1;
  }
  int ans = 0;
  for (int x1 = 0; x1 < r; x1++)
    for (int y1 = 0; y1 < c; y1++)
      for (int x2 = x1; x2 < r; x2++)
        for (int y2 = y1; y2 < c; y2++) {
          int cmp = 0;
          for (int i = x1; i <= x2; i++)
            for (int j = y1; j <= y2; j++) cmp += t[i][j];
          ans += (cmp >= k);
        }
  cout << ans << '\n';
  return 0;
}
