#include <bits/stdc++.h>
using namespace std;
int a[12][12];
int com[12][12];
int main() {
  int r, c, n, k, sum = 0;
  cin >> r >> c >> n >> k;
  for (int i = 0; i < n; i++) {
    int x, y;
    cin >> x >> y;
    a[x][y] = 1;
  }
  for (int i = 1; i <= r; i++) {
    for (int j = 1; j <= c; j++) {
      com[i][j] = a[i][j] + com[i - 1][j] + com[i][j - 1] - com[i - 1][j - 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++) {
          if (com[x][y] - com[i - 1][y] - com[x][j - 1] + com[i - 1][j - 1] >=
              k)
            sum++;
        }
      }
    }
  }
  cout << sum << endl;
}
