#include <bits/stdc++.h>
using namespace std;
int a[12][12], s[12][12];
int main() {
  int r, c, n, k, x, y;
  cin >> r >> c >> n >> k;
  for (int i = 1; i <= n; i++) {
    cin >> x >> y;
    a[x][y] = 1;
  }
  int ans = 0;
  for (int i = 1; i <= r; i++)
    for (int j = 1; j <= c; j++)
      for (int i1 = i; i1 <= r; i1++)
        for (int j1 = j; j1 <= c; j1++) {
          int cnt = 0;
          for (int i2 = i; i2 <= i1; i2++)
            for (int j2 = j; j2 <= j1; j2++) cnt += a[i2][j2];
          if (cnt >= k) ans++;
        }
  cout << ans;
  return 0;
}
