#include <bits/stdc++.h>
using namespace std;
int n, k, r, c;
vector<string> grip;
int main() {
  grip.clear();
  cin >> r >> c >> n >> k;
  string ful = "";
  for (int i = 0; i < c; i++) ful += '.';
  for (int i = 0; i < r; i++) grip.push_back(ful);
  for (int i = 0; i < n; i++) {
    int y, x;
    cin >> y >> x;
    y--, x--;
    grip[y][x] = '#';
  }
  int ans = 0;
  for (int il = 0; il < r; il++) {
    for (int ir = il; ir < r; ir++) {
      for (int jl = 0; jl < c; jl++) {
        for (int jr = jl; jr < c; jr++) {
          int con = 0;
          for (int p = il; p <= ir; p++) {
            for (int q = jl; q <= jr; q++)
              if (grip[p][q] == '#') con++;
          }
          if (con >= k) ans++;
        }
      }
    }
  }
  cout << ans << endl;
}
