#include <bits/stdc++.h>
using namespace std;
int arr[10][10];
int main() {
  int row, col, n, num, r, c;
  cin >> row >> col >> n >> num;
  for (int i = 0; i < n; i++) {
    cin >> r >> c;
    arr[r - 1][c - 1] = 1;
  }
  for (int i = 0; i < row; i++) {
    for (int j = 0; j < col; j++) {
      if (i) arr[i][j] += arr[i - 1][j];
      if (j) arr[i][j] += arr[i][j - 1];
      if (i && j) arr[i][j] -= arr[i - 1][j - 1];
    }
  }
  int sum = 0, ans = 0;
  for (int i = 0; i < row; i++) {
    for (int j = 0; j < col; j++) {
      for (int k = 0; k <= i; k++) {
        for (int m = 0; m <= j; m++) {
          sum = arr[i][j];
          if (k) sum -= arr[k - 1][j];
          if (m) sum -= arr[i][m - 1];
          if (m && k) sum += arr[k - 1][m - 1];
          if (sum >= num) {
            ans++;
          }
        }
      }
    }
  }
  cout << ans << endl;
  return 0;
}
