#include <bits/stdc++.h>
using namespace std;
int v[15][15], i, j, x, y, r, c, k, n, nr;
int main() {
  cin >> r >> c >> n >> k;
  for (i = 1; i <= n; i++) {
    cin >> x >> y;
    v[x][y] = 1;
  }
  for (i = 1; i <= r; i++)
    for (j = 1; j <= c; j++)
      v[i][j] = v[i][j] + v[i - 1][j] + v[i][j - 1] - v[i - 1][j - 1];
  for (x = 1; x <= r; x++)
    for (y = 1; y <= c; y++)
      if (x * y >= k) {
        for (i = 1; i <= r - x + 1; i++)
          for (j = 1; j <= c - y + 1; j++) {
            if (v[i + x - 1][j + y - 1] + v[i - 1][j - 1] -
                    v[i + x - 1][j - 1] - v[i - 1][j + y - 1] >=
                k)
              nr++;
          }
      }
  cout << nr;
}
