#include <bits/stdc++.h>
using namespace std;
int calc(const vector<vector<int>>& grid, int a, int b, int c, int d) {
  int res = 0;
  for (int i = a; i < b; ++i) {
    for (int j = c; j < d; ++j) {
      res += grid[i][j];
    }
  }
  return res;
}
int main() {
  ios::sync_with_stdio(false);
  cin.tie(nullptr);
  int r, c, n, k;
  cin >> r >> c >> n >> k;
  vector<vector<int>> grid(r, vector<int>(c, 0));
  for (int i = 0; i < n; ++i) {
    int x, y;
    cin >> x >> y;
    grid[x - 1][y - 1] = 1;
  }
  int ans = 0;
  for (int i = 0; i < r; ++i) {
    for (int j = 0; j < c; ++j) {
      for (int ii = i + 1; ii <= r; ++ii) {
        for (int jj = j + 1; jj <= c; ++jj) {
          if (calc(grid, i, ii, j, jj) >= k) {
            ans += 1;
          }
        }
      }
    }
  }
  cout << ans << endl;
}
