#include <bits/stdc++.h>
using namespace std;
int main() {
  int r, c, n, mnm, x, y, ret = 0;
  cin >> r >> c >> n >> mnm;
  vector<vector<int> > mapa(r, vector<int>(c, 1));
  for (int i = 0; i < n; i++) {
    cin >> x >> y;
    mapa[x - 1][y - 1] = 2;
  }
  for (int i = 1; i <= r; i++) {
    for (int j = 1; j <= c; j++) {
      for (int k = 0; k < r - (i - 1); k++) {
        for (int l = 0; l < c - (j - 1); l++) {
          int cnt = 0;
          for (int m = 0; m < i; m++) {
            for (int n = 0; n < j; n++) {
              if (mapa[k + m][l + n] == 2) cnt++;
            }
          }
          if (cnt >= mnm) ret++;
        }
      }
    }
  }
  cout << ret << endl;
}
