#include <bits/stdc++.h>
using namespace std;
const int MAXN = 11;
int N, M, total, k, a, b;
bool vis[MAXN][MAXN];
int cnt(int r1, int c1, int r2, int c2) {
  int ans = 0;
  for (int i = r1; i <= r2; i++)
    for (int j = c1; j <= c2; j++)
      if (vis[i][j]) ans++;
  return ans;
}
int solve() {
  int ans = 0;
  for (int i = 0; i < N; i++)
    for (int j = 0; j < M; j++)
      for (int ii = 0; ii < N; ii++)
        for (int jj = 0; jj < M; jj++)
          if (cnt(i, j, ii, jj) >= k) ans++;
  return ans;
}
int main() {
  scanf("%d %d %d %d", &N, &M, &total, &k);
  for (int i = 0; i < total; i++) {
    scanf("%d %d", &a, &b);
    a--, b--;
    vis[a][b] = true;
  }
  cout << solve() << endl;
  return 0;
}
