#include <bits/stdc++.h>
using namespace std;
int main(int argc, const char* argv[]) {
  int r, c, n, p;
  cin >> r >> c >> n >> p;
  int a[r + 1][c + 1];
  int d[r + 1][c + 1];
  for (int i = 0; i < r + 1; i++) {
    for (int j = 0; j < c + 1; j++) {
      a[i][j] = 0;
      d[i][j] = 0;
    }
  }
  int x, y;
  for (int i = 0; i < n; i++) {
    cin >> x >> y;
    a[x][y] = 1;
  }
  for (int i = 1; i < r + 1; i++) {
    for (int j = 1; j < c + 1; j++) {
      d[i][j] = a[i][j] + d[i][j - 1] + d[i - 1][j] - d[i - 1][j - 1];
    }
  }
  int ans = 0;
  for (int i = 1; i < r + 1; i++) {
    for (int j = 1; j < c + 1; j++) {
      for (int t = 1; t < r + 1; t++) {
        for (int k = 1; k < c + 1; k++) {
          int sum = 0;
          for (int q = i; q <= t; q++) {
            for (int w = j; w <= k; w++) {
              sum += a[q][w];
            }
          }
          if (sum >= p) {
            ans++;
          }
        }
      }
    }
  }
  cout << ans;
}
