#include <bits/stdc++.h>
using namespace std;
int mark[20][20], r, c, n, k, ans;
int main() {
  cin >> r >> c >> n >> k;
  for (int i = 0; i < n; i++) {
    int a, b;
    cin >> a >> b;
    a--, b--;
    mark[a][b] = 1;
  }
  for (int i = 0; i < r; i++)
    for (int ii = i; ii < r; ii++)
      for (int j = 0; j < c; j++)
        for (int jj = j; jj < c; jj++) {
          int res = 0;
          for (int k = i; k <= ii; k++)
            for (int l = j; l <= jj; l++) res += mark[k][l];
          if (res >= k) ans++;
        }
  cout << ans << endl;
}
