#include <bits/stdc++.h>
using namespace std;
int g[12][12];
int main(void) {
  ios_base::sync_with_stdio(false);
  int n, r, c, k, res = 0;
  cin >> r >> c >> n >> k;
  int a, b;
  for (int i = 0; i < n; i++) {
    cin >> a >> b;
    g[a][b] = 1;
  }
  for (int i = 1; i <= r; i++) {
    for (int j = 1; j <= c; j++) {
      for (int x = i; x <= r; x++) {
        for (int y = j; y <= c; y++) {
          int cont = 0;
          for (int l = i; l <= x; l++) {
            for (int m = j; m <= y; m++) {
              if (g[l][m] == 1) cont++;
            }
          }
          if (cont >= k) res++;
        }
      }
    }
  }
  cout << res << endl;
  return 0;
}
