#include <bits/stdc++.h>
using namespace std;
int mi[] = {0, -1, 0, 1, -1, 1, 1, -1};
int mj[] = {1, 0, -1, 0, 1, 1, -1, -1};
int mat[11][11];
void init() {
  freopen("in.txt", "r", stdin);
  freopen("out.txt", "w", stdout);
  cout << "[FREOPEN]" << endl;
  return;
}
int main() {
  int n, m, c, k;
  scanf("%d %d %d %d", &n, &m, &c, &k);
  memset(mat, 0, sizeof(mat));
  while (c--) {
    int x, y;
    scanf("%d %d", &x, &y);
    x--, y--;
    mat[x][y] = 1;
  }
  int ans = 0;
  for (int i = 0; i < n; i++) {
    for (int j = 0; j < m; j++) {
      for (int ii = i; ii < n; ii++) {
        for (int jj = j; jj < m; jj++) {
          int cnt = 0;
          for (int iii = i; iii <= ii; iii++) {
            for (int jjj = j; jjj <= jj; jjj++) {
              cnt += mat[iii][jjj];
            }
          }
          ans += (cnt >= k);
        }
      }
    }
  }
  printf("%d\n", ans);
  return 0;
}
