#include <bits/stdc++.h>
using namespace std;
bool a[20][20];
int main() {
  ios::sync_with_stdio(false);
  cin.tie(0);
  cout.tie(0);
  int r, c, n, k;
  cin >> r >> c >> n >> k;
  for (int i = 0; i < n; i++) {
    int x, y;
    cin >> x >> y;
    a[x][y] = 1;
  }
  int ans = 0;
  for (int i = 1; i <= r; i++) {
    for (int j = i; j <= r; j++) {
      for (int m = 1; m <= c; m++) {
        for (int l = m; l <= c; l++) {
          int cnt = 0;
          for (int a1 = i; a1 <= j; a1++) {
            for (int a2 = m; a2 <= l; a2++) {
              if (a[a1][a2] == 1) {
                cnt++;
              }
            }
          }
          if (cnt >= k) {
            ans++;
          }
        }
      }
    }
  }
  cout << ans;
  return 0;
}
