#include <bits/stdc++.h>
using namespace std;
const int inf = (int)1e9;
const int mod = inf + 7;
const double eps = 1e-9;
const double pi = acos(-1.0);
int r, c, n, k, ans, used[1000][1000];
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(0);
  cin >> r >> c >> n >> k;
  for (int i = 0; i < n; ++i) {
    int x, y;
    cin >> x >> y;
    used[x][y] = 1;
  }
  for (int i = 1; i <= r; ++i) {
    for (int j = 1; j <= c; ++j) {
      for (int t = i; t <= r; ++t) {
        for (int l = j; l <= c; ++l) {
          int cnt = 0;
          for (int v = i; v <= t; ++v) {
            for (int m = j; m <= l; ++m) {
              if (used[v][m]) cnt++;
            }
          }
          if (cnt >= k) ans++;
        }
      }
    }
  }
  cout << ans << endl;
}
