#include <bits/stdc++.h>
using namespace std;
int m[11][11];
int main() {
  int r, c, n, k, x, y;
  memset(m, 0, sizeof m);
  cin >> r >> c >> n >> k;
  for (int i = 1; i <= n; ++i) {
    cin >> x >> y;
    m[x][y] = 1;
  }
  int ans = 0;
  for (int xi = 1; xi <= r; ++xi) {
    for (int yi = 1; yi <= c; ++yi) {
      for (int xj = xi; xj <= r; ++xj) {
        for (int yj = yi; yj <= c; ++yj) {
          int x = 0;
          for (int i = xi; i <= xj; ++i)
            for (int j = yi; j <= yj; ++j) x += m[i][j];
          if (x >= k) ++ans;
        }
      }
    }
  }
  cout << ans << "\n";
  return 0;
}
