#include <bits/stdc++.h>
using namespace std;
int mp[20][20];
int ct[20][20];
int main() {
  memset(mp, 0, sizeof mp);
  memset(ct, 0, sizeof ct);
  int r, c, n, m, ans = 0;
  cin >> r >> c >> n >> m;
  for (int i = 1; i <= n; i++) {
    int x, y;
    cin >> x >> y;
    mp[x][y] = 1;
  }
  for (int i = 1; i <= r; i++) {
    for (int j = 1; j <= c; j++) {
      ct[i][j] = ct[i - 1][j] + ct[i][j - 1] - ct[i - 1][j - 1] + mp[i][j];
    }
  }
  for (int i = 1; i <= r; i++) {
    for (int j = 1; j <= c; j++) {
      for (int k = i; k <= r; k++) {
        for (int l = j; l <= c; l++) {
          int cnt = ct[k][l] + ct[i - 1][j - 1] - ct[i - 1][l] - ct[k][j - 1];
          if (cnt >= m) ans++;
        }
      }
    }
  }
  cout << ans << endl;
  return 0;
}
