#include <bits/stdc++.h>
using namespace std;
int main() {
  int r, c, n, m, k, i, j, cnt, x, y, z, b;
  cin >> r >> c >> n >> k;
  char a[r][c];
  for (i = 0; i < r; ++i) {
    for (j = 0; j < c; ++j) a[i][j] = '*';
  }
  m = n;
  while (m--) {
    cin >> i >> j;
    a[i - 1][j - 1] = '#';
  }
  cnt = 0;
  for (i = 0; i < r; ++i) {
    for (j = 0; j < c; ++j) {
      for (x = i; x < r; ++x) {
        for (y = j; y < c; ++y) {
          m = 0;
          for (z = i; z <= x; ++z) {
            for (b = j; b <= y; ++b) {
              if (a[z][b] == '#') ++m;
            }
          }
          if (m >= k) ++cnt;
        }
      }
    }
  }
  cout << cnt;
  return 0;
}
