#include <bits/stdc++.h>
using namespace std;
const int nmax = 2000100;
int n, a[50][50], r, c, k, ans;
int main() {
  ios::sync_with_stdio(0);
  cin >> r >> c >> n >> k;
  for (int i = 0; i < n; i++) {
    int l, r;
    cin >> l >> r;
    a[l][r] = 1;
  }
  int cnt = 0;
  for (int i = 1; i <= r; i++)
    for (int j = 1; j <= c; j++)
      for (int i2 = i; i2 <= r; i2++)
        for (int j2 = j; j2 <= c; j2++) {
          int s = 0;
          for (int i3 = i; i3 <= i2; i3++)
            for (int j3 = j; j3 <= j2; j3++) s += a[i3][j3];
          if (s >= k) cnt++;
        }
  cout << cnt << endl;
}
