#include <bits/stdc++.h>
using namespace std;
const long long linf = 1e18 + 9;
const int inf = 1e9 + 9;
const int N = 1e2 + 9;
long long r, c, n, k, a, b, arr[N][N], ans, res;
int main() {
  cin >> r >> c >> n >> k;
  for (int i = 1; i <= n; i++) {
    cin >> a >> b;
    arr[a][b] = 1;
  }
  for (int i = 1; i <= r; i++) {
    for (int j = i; j <= r; j++) {
      for (int t = 1; t <= c; t++) {
        for (int t2 = t; t2 <= c; t2++) {
          for (int rec = i; rec <= j; rec++) {
            for (int rec2 = t; rec2 <= t2; rec2++) {
              if (arr[rec][rec2]) {
                ans++;
              }
            }
          }
          if (ans >= k) {
            res++;
          }
          ans = 0;
        }
      }
    }
  }
  cout << res << endl;
  return 0;
}
