#include <bits/stdc++.h>
using namespace std;
bool v[11][11];
int main() {
  int r, c, n, k;
  cin >> r >> c >> n >> k;
  for (long long i = 0; i < n; i++) {
    int x, y;
    cin >> x >> y;
    v[x - 1][y - 1] = true;
  }
  int g_count = 0;
  for (long long i = 0; i < r; i++) {
    for (long long j = 0; j < c; j++) {
      for (long long z = 0; z < r; z++) {
        for (long long q = 0; q < c; q++) {
          int l_count = 0;
          if (i + z < r && j + q < c)
            for (long long m = i; m < i + z + 1; m++) {
              for (long long g = j; g < j + q + 1; g++) {
                if (v[m][g]) l_count++;
              }
            }
          if (l_count >= k) g_count++;
        }
      }
    }
  }
  cout << g_count << endl;
}
